Semiconductor Device, Manufacturing Method of the Same, and Electronic Device

ABSTRACT

A semiconductor device in which parasitic capacitance is reduced is provided. A first insulating layer is deposited over a substrate. A first oxide insulating layer and an oxide semiconductor layer are deposited over the first insulating layer. A second oxide insulating layer is deposited over the oxide semiconductor layer and the first insulating layer. A second insulating layer and a first conductive layer are deposited over the second oxide insulating layer. A gate electrode layer, a gate insulating layer, and a third oxide insulating layer are formed by etching. A sidewall insulating layer including a region in contact with a side surface of the gate electrode layer is formed. A second conductive layer is deposited over the gate electrode layer, the sidewall insulating layer, the oxide semiconductor layer, and the first insulating layer. A third conductive layer is deposited over the second conductive layer. A low-resistance region is formed in the oxide semiconductor layer by performing heat treatment. An element contained in the second conductive layer moves from the second conductive layer to the oxide semiconductor layer side by performing the heat treatment. An element contained in the oxide semiconductor layer moves from the oxide semiconductor layer to the third conductive layer side by performing the heat treatment.

This application is a divisional of copending U.S. application Ser. No.15/224,958, filed on Aug. 1, 2016 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. In addition, the present invention relates to a process, amachine, manufacture, or a composition of matter. In particular, thepresent invention relates to, for example, a semiconductor device, adisplay device, a light-emitting device, a power storage device, animaging device, a driving method thereof, or a manufacturing methodthereof. In particular, one embodiment of the present invention relatesto a semiconductor device or a method for manufacturing thesemiconductor device.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a memory device, adisplay device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

A technique by which a transistor is formed using a semiconductor filmformed over a substrate having an insulating surface has been attractingattention. The transistor is used in a wide range of electronic devicessuch as an integrated circuit (IC) or an image display device (displaydevice). A silicon-based semiconductor material is widely known as amaterial for a semiconductor thin film that can be used for atransistor. As another material, an oxide semiconductor has beenattracting attention.

For example, a transistor whose active layer includes an amorphous oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn) isdisclosed in Patent Document 1.

A method for manufacturing a transistor in which a conductive layer isformed over an oxide semiconductor layer and heat treatment is performedto reduce resistance of the oxide semiconductor layer are disclosed inPatent Document 2 and Non-Patent Document.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2013-175710

Non-Patent Document

-   [Non-Patent Document 1] 2015 Symposium on VLSI Technology Digest of    Technical Papers, T214-T215.

SUMMARY OF THE INVENTION

Manufacturing of semiconductor devices with highly integratedtransistors requires miniaturization of transistors. However, inminiaturization of transistors, a remarkable increase in the parasiticcapacitance of the transistors may cause a problem.

For example, in the case where parasitic capacitance exists in a channel(e.g., between a source electrode and a drain electrode) and thevicinity of the channel, a time for charging the parasitic capacitanceis needed in the transistor operation; thus, not only the responsivenessof the transistor but the responsiveness of the semiconductor device islowered.

It becomes more difficult to control various steps of manufacturingtransistors (in particular, film formation, processing, and the like) asthe miniaturization advances. Thus, variations due to a manufacturingprocess significantly affect transistor characteristics and reliability.

Thus, an object of one embodiment of the present invention is to reducethe parasitic capacitance in a transistor and in the vicinity of thetransistor. Another object is to provide a semiconductor device capableof high-speed operation. Another object is to provide a semiconductordevice with favorable electrical characteristics. Another object is toprovide a semiconductor device with high reliability. Another object isto reduce variations in characteristics of a semiconductor device thatare caused by a manufacturing process. Another object is to provide asemiconductor device including an oxide semiconductor layer having fewoxygen vacancies. Another object is to provide a semiconductor devicethat can be manufactured in a simple process. Another object is toprovide a semiconductor device in which the amount of interface statesin the vicinity of the oxide semiconductor layer can be reduced. Anotherobject is to provide a semiconductor device with low power consumption.Further, another object is to provide a novel semiconductor device orthe like. Another object is to provide a manufacturing method of thesemiconductor device.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

(1) One embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of: depositinga first insulating layer over a substrate; sequentially depositing afirst oxide insulating layer and a first oxide semiconductor layer overthe first insulating layer; forming a second oxide insulating layer anda second oxide semiconductor layer by etching the first oxide insulatinglayer and the first oxide semiconductor layer into island shapes with afirst mask; depositing a third oxide insulating layer over the secondoxide semiconductor layer and the first insulating layer; depositing asecond insulating layer over the third oxide insulating layer;depositing a first conductive layer over the second insulating layer;forming a gate electrode layer and a gate insulating layer byselectively etching the first conductive layer and the second insulatinglayer with a second mask; depositing a third insulating layer over thegate electrode layer, the second oxide semiconductor layer, and thefirst insulating layer; forming a sidewall insulating layer including aregion in contact with a side surface of the gate electrode layer byperforming dry etching on the third insulating layer; depositing asecond conductive layer over the gate electrode layer, the sidewallinsulating layer, the second oxide semiconductor layer, and the firstinsulating layer; depositing a third conductive layer over the secondconductive layer; and forming a low-resistance region in the secondoxide semiconductor layer by performing heat treatment. An elementcontained in the second conductive layer moves from the secondconductive layer to the second oxide semiconductor layer side byperforming the heat treatment. An element contained in the second oxidesemiconductor layer moves from the second oxide semiconductor layer tothe third conductive layer side by performing the heat treatment.

(2) One embodiment of the present invention is a method formanufacturing a semiconductor device including the steps of: depositinga first insulating layer over a substrate; sequentially depositing afirst oxide insulating layer and a first oxide semiconductor layer overthe first insulating layer; depositing a second oxide insulating layerand a second oxide semiconductor layer by performing etching on thefirst oxide insulating layer and the first oxide semiconductor layerinto island shapes with a first mask; depositing a third oxideinsulating layer over the second oxide semiconductor layer and the firstinsulating layer; depositing a second insulating layer over the thirdoxide insulating layer; forming a third insulating layer by performingplanarization treatment on the second oxide insulating layer; forming afourth insulating layer with a groove portion that reaches the thirdoxide insulating layer by etching part of the third insulating layerwith a second mask; depositing a fifth insulating layer over the fourthinsulating layer and the third oxide insulating layer; depositing afirst conductive layer over the fifth insulating layer; forming a gateelectrode layer and a sixth insulating layer by performing planarizationtreatment on the first conductive layer and the fifth insulating layeruntil the fourth insulating layer is exposed; forming a gate insulatinglayer by etching the fourth insulating layer and the sixth insulatinglayer with the gate electrode layer as a mask; depositing a seventhinsulating layer over the gate electrode layer, the second oxidesemiconductor layer, and the first insulating layer; forming a sidewallinsulating layer including a region in contact with a side surface ofthe gate electrode layer by performing dry etching on the seventhinsulating layer; depositing a second conductive layer over the gateelectrode layer, the sidewall insulating layer, the second oxidesemiconductor layer, and the first insulating layer; depositing a thirdconductive layer over the second conductive layer; and forming alow-resistance region in the second oxide semiconductor layer byperforming heat treatment. An element contained in the second conductivelayer moves from the second conductive layer to the second oxidesemiconductor layer side by performing the heat treatment. An elementcontained in the second oxide semiconductor layer moves from the secondoxide semiconductor layer to the third conductive layer side byperforming the heat treatment.

(3) It is preferable that, in the above method for manufacturing asemiconductor device, the second conductive layer comprises cobalt,nickel, platinum, vanadium, chromium, or palladium, and the thirdconductive layer contains titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, rhodium, iridium, orcobalt.

(4) It is preferable that, in the above method for manufacturing asemiconductor device, the low-resistance region includes a regioncontaining an alloy.

(5) It is preferable that, in the above method for manufacturing asemiconductor device, the heat treatment is performed at a temperaturehigher than or equal to 450° C. and lower than or equal to 650° C.

(6) Another embodiment of the present invention is a semiconductordevice including a first insulating layer over a substrate; a firstoxide insulating layer over the first insulating layer; an oxidesemiconductor layer over the first oxide insulating layer; a secondoxide insulating layer over the oxide semiconductor layer; a gateinsulating layer over the second oxide insulating layer; a gateelectrode layer over the gate insulating layer; and a sidewallinsulating layer over the oxide semiconductor layer. A bottom surface ofthe oxide semiconductor layer is located above a bottom surface of thegate electrode layer in a cross section in a channel width direction. Atop surface and a side surface of the oxide semiconductor layer eachinclude a region facing the gate electrode layer. The sidewallinsulating layer includes a region in contact with a side surface of thegate insulating layer. The oxide semiconductor layer includes first tothird regions. The first region includes a region overlapping with thegate electrode layer. The second region includes a region which isbetween the first region and the third region and overlaps with thesidewall insulating layer. The second region includes a region withlower resistance than the first region. The third region includes aregion with lower resistance than the second region. The second regionand the third region each include any one of elements of cobalt, nickel,vanadium, and palladium, and any one of elements of titanium, zirconium,hafnium, and tantalum.

(7) It is preferable that, in the above semiconductor device, the secondregion and the third region each include a region containing an alloy.

(8) Another embodiment of the present invention is an electronic deviceincluding the semiconductor device, a housing, and a speaker.

According to one embodiment of the present invention, the parasiticcapacitance in a transistor and in the vicinity of the transistor can bereduced, and a semiconductor device capable of high-speed operation canbe provided. Alternatively, a semiconductor device with favorableelectrical characteristics can be provided. Alternatively, asemiconductor device with high reliability can be provided.Alternatively, variations in characteristics of a transistor or asemiconductor device that are caused by a manufacturing process can bereduced. Alternatively, a semiconductor device including an oxidesemiconductor layer having few oxygen vacancies can be provided.Alternatively, a semiconductor device that can be manufactured in asimple process can be provided. Alternatively, a semiconductor devicewith a structure in which the density of interface states in and near anoxide semiconductor layer can be reduced can be provided. Alternatively,a semiconductor device with low power consumption can be provided.Alternatively, a novel semiconductor device or the like can be provided.Alternatively, a method for manufacturing the semiconductor device canbe provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects listed above. Other effects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views which illustratea transistor.

FIGS. 2A to 2C are a top view and cross-sectional views illustrating atransistor.

FIGS. 3A and 3B are a cross-sectional view of a transistor and a banddiagram of an oxide semiconductor layer.

FIGS. 4A to 4D illustrate an ALD mechanism.

FIGS. 5A and 5B are schematic views of an ALD apparatus.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 7A to 7C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 8A to 8C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 9A to 9C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 10A to 10C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 11A to 11C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 12A to 12C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 13A to 13C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 14A to 14C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 15A to 15C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 16A to 16C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 17A to 17C are a top view and cross-sectional views illustratingthe method for manufacturing a transistor.

FIGS. 18A to 18C are a top view and cross-sectional views illustrating amethod for manufacturing a transistor.

FIGS. 19A to 19C are a top view and cross-sectional views illustrating atransistor.

FIGS. 20A to 20C are a top view and cross-sectional views of atransistor.

FIGS. 21A to 21C are a top view and cross-sectional views of atransistor.

FIGS. 22A to 22C are a top view and cross-sectional views of atransistor.

FIGS. 23A to 23C are a top view and cross-sectional views of atransistor.

FIGS. 24A to 24E are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 25A to 25E are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS.

FIGS. 26A to 26D show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor layer by XRD.

FIGS. 27A and 27B show electron diffraction patterns of a CAAC-OS.

FIG. 28 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

FIGS. 29A to 29D are cross-sectional views and circuit diagrams of asemiconductor device.

FIGS. 30A to 30C are a cross-sectional view and circuit diagrams of asemiconductor device.

FIGS. 31A and 31B are plan views of an imaging device.

FIGS. 32A and 32B are plan views of pixels of an imaging device.

FIGS. 33A and 33B are cross-sectional views of an imaging device.

FIGS. 34A and 34B are cross-sectional views of an imaging device.

FIGS. 35A to 35C are circuit diagrams and a timing chart illustrating asemiconductor device of one embodiment of the present invention.

FIGS. 36A to 36C are a graph and circuit diagrams illustrating asemiconductor device of one embodiment of the present invention.

FIGS. 37A and 37B are a circuit diagram and a timing chart illustratinga semiconductor device of one embodiment of the present invention.

FIGS. 38A and 38B are a circuit diagram and a timing chart illustratinga semiconductor device of one embodiment of the present invention.

FIGS. 39A to 39E are a block diagram, circuit diagrams, and waveformdiagrams for illustrating one embodiment of the present invention.

FIGS. 40A and 40B are a circuit diagram and a timing chart illustratingone embodiment of the present invention.

FIGS. 41A and 41B are circuit diagrams for illustrating one embodimentof the present invention.

FIGS. 42A to 42C are circuit diagrams each illustrating one embodimentof the present invention.

FIG. 43 illustrates a configuration example of an RF tag.

FIG. 44 illustrates a configuration example of a CPU.

FIG. 45 is a circuit diagram of a memory element.

FIGS. 46A to 46C illustrate a configuration example of a display deviceand circuit diagrams of pixels.

FIGS. 47A and 47B are a top view and a cross-sectional view of a liquidcrystal display device.

FIGS. 48A and 48B are a top view and a cross-sectional view of alight-emitting device.

FIG. 49 illustrates a display module.

FIG. 50A is a perspective view illustrating a cross-sectional structureof a package using a lead frame interposer, and FIG. 50B is a plan viewillustrating a structure of a module of a mobile phone.

FIGS. 51A to 51E illustrate electronic devices.

FIGS. 52A to 52D illustrate electronic devices.

FIGS. 53A to 53C illustrate electronic devices.

FIGS. 54A to 54F illustrate electronic devices.

FIG. 55 is a cross-sectional view illustrating a method formanufacturing a transistor.

FIG. 56 is a schematic cross-sectional view illustrating an evaluationsample.

FIGS. 57A and 57B illustrate cross-sectional STEM observation results ofevaluation samples and EDX analysis results thereof.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription. It will be readily appreciated by those skilled in the artthat modes and details of the present invention can be modified invarious ways without departing from the spirit and scope of the presentinvention. The present invention therefore should not be construed asbeing limited to the following description of the embodiments. Instructures of the invention described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and the description thereof is notrepeated in some cases. The same components are denoted by differenthatching patterns in different drawings, or the hatching patterns areomitted in some cases.

<Notes on Description for Drawings>

In this specification, terms for describing arrangement, such as “over”and “under,” are used for convenience to describe a positional relationbetween components with reference to drawings. Furthermore, thepositional relation between components is changed as appropriate inaccordance with a direction in which each component is described. Thus,there is no limitation on terms used in this specification, anddescription can be made appropriately depending on the situation.

The term “over” or “under” does not necessarily mean that a component isplaced directly over or directly under and directly in contact withanother component. For example, the expression “electrode B overinsulating layer A” does not necessarily mean that the electrode B isover and in direct contact with the insulating layer A and can mean thecase where another component is provided between the insulating layer Aand the electrode B.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

In drawings, the size, the layer thickness, or the region is determinedarbitrarily for description convenience. Therefore, the size, the layerthickness, or the region is not limited to the illustrated scale. Notethat the drawings are schematically shown for clarity, and embodimentsof the present invention are not limited to shapes or values shown inthe drawings.

In drawings such as plan views (also referred to as layout views) andperspective views, some of components might not be illustrated forclarity of the drawings.

<Notes about the Description that can be Rephrased or Reworded>

In this specification, in description of connections of a transistor,expressions “one of a source and a drain” (or a first electrode or afirst terminal) and “the other of the source and the drain” (or a secondelectrode or a second terminal) are used. This is because a source and adrain of a transistor are interchangeable depending on the structure,operation conditions, or the like of the transistor. Note that thesource or the drain of the transistor can also be referred to as asource (or drain) terminal, a source (or drain) electrode, or the likeas appropriate depending on the situation.

In this specification, the term of “electrode” or “wiring” does notlimit the function of components. For example, an “electrode” issometimes used as part of a “wiring”, and vice versa. Furthermore, theterm “electrode” or “wiring” can include the case where a plurality of“electrodes” or “wirings” is formed in an integrated manner.

In this specification, a transistor is an element having at least threeterminals of a gate, a drain, and a source. The transistor includes achannel region between the drain (a drain terminal, a drain region, or adrain electrode) and the source (a source terminal, a source region, ora source electrode) and current can flow through the drain, the channelregion, and the source.

Here, since the source and the drain of the transistor change dependingon the structure, the operating condition, and the like of thetransistor, it is difficult to define which is a source or a drain.Thus, a portion that functions as a source or a portion that functionsas a drain is not referred to as a source or a drain in some cases. Inthat case, one of the source and the drain might be referred to as afirst electrode, and the other of the source and the drain might bereferred to as a second electrode.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In this specification, the terms “film” and “layer” can be interchangeddepending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

The expression “being the same” in this specification may refer tohaving the same area or having the same shape. In addition, theexpression “being the same” include a case of “being substantially thesame” because a manufacturing process might cause some differences.

Note that what is described (or part thereof) in an embodiment in thisspecification can be applied to, combined with, or replaced with anothercontent in the same embodiment and/or what is described (or partthereof) in another embodiment or other embodiments.

Note that by combining a diagram (or may be part of the diagram)illustrated in one embodiment in this specification with another part ofthe diagram, a different diagram (or may be part of the differentdiagram) illustrated in the embodiment, and/or a diagram (or may be partof the diagram) illustrated in one or a plurality of differentembodiments, much more diagrams can be formed.

<Notes on Definitions of Terms and Other Matters>

The following are definitions of the terms mentioned in the aboveembodiments and other matters.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 100, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.A term “substantially parallel” indicates that the angle formed betweentwo straight lines is greater than or equal to −30° and less than orequal to 30°. Furthermore, the term “perpendicular” indicates that theangle formed between two straight lines is greater than or equal to 80°and less than or equal to 100°, and accordingly also includes the casewhere the angle is greater than or equal to 85° and less than or equalto 95°. In addition, the term “substantially perpendicular” indicatesthat an angle formed between two straight lines is greater than or equalto 60° and less than or equal to 120°.

In this specification, the term “trench” or “groove” refers to adepression with a narrow belt shape.

Furthermore, in this specification, an explicit description “X and Y areconnected” means that X and Y are electrically connected, X and Y arefunctionally connected, and X and Y are directly connected. Accordingly,another element may be provided between elements having a connectionrelation illustrated in drawings and texts, without being limited to apredetermined connection relation, for example, the connection relationillustrated in the drawings and the texts.

Here, each of X and Y denotes an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer).

Examples of the case where X and Y are directly connected include thecase where an element that enables electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) is notconnected between X and Y, and the case where X and Y are connectedwithout the element that enables electrical connection between X and Yprovided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off. That is, the switch is conducting or not conducting(is turned on or off) to determine whether current flows therethrough ornot. Alternatively, the switch has a function of selecting and changinga current path. Note that the case where X and Y are electricallyconnected includes the case where X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a D/A converter circuit, anA/D converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; or a control circuit) can be connected between X and Y.Note that for example, in the case where a signal output from X istransmitted to Y even when another circuit is provided between X and Y,X and Y are functionally connected. The case where X and Y arefunctionally connected includes the case where X and Y are directlyconnected and X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the explicit description “X and Yare connected”.

For example, the case where a source (or a first terminal or the like)of a transistor is electrically connected to X through (or not through)Z1 and a drain (or a second terminal or the like) of the transistor iselectrically connected to Y through (or not through) Z2, or the casewhere a source (or a first terminal or the like) of a transistor isdirectly connected to part of Z1 and another part of Z1 is directlyconnected to X while a drain (or a second terminal or the like) of thetransistor is directly connected to part of Z2 and another part of Z2 isdirectly connected to Y, can be expressed by using any of the followingexpressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in that order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin that order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are connected in that order”. When theconnection order in a circuit configuration is defined by an expressionsimilar to the above examples, a source (or a first terminal or thelike) and a drain (or a second terminal or the like) of a transistor canbe distinguished from each other to specify the technical scope.

Other examples of the expressions include “a source (or a first terminalor the like) of a transistor is electrically connected to X through atleast a first connection path, the first connection path does notinclude a second connection path, the second connection path is a pathbetween the source (or the first terminal or the like) of the transistorand a drain (or a second terminal or the like) of the transistor, Z1 ison the first connection path, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leasta third connection path, the third connection path does not include thesecond connection path, and Z2 is on the third connection path”. It isalso possible to use the expression “a source (or a first terminal orthe like) of a transistor is electrically connected to X through atleast Z1 on a first connection path, the first connection path does notinclude a second connection path, the second connection path includes aconnection path through the transistor, a drain (or a second terminal orthe like) of the transistor is electrically connected to Y through atleast Z2 on a third connection path, and the third connection path doesnot include the second connection path”. Still another example of theexpressions is “a source (or a first terminal or the like) of atransistor is electrically connected to X through at least Z1 on a firstelectrical path, the first electrical path does not include a secondelectrical path, the second electrical path is an electrical path fromthe source (or the first terminal or the like) of the transistor to adrain (or a second terminal or the like) of the transistor, the drain(or the second terminal or the like) of the transistor is electricallyconnected to Y through at least Z2 on a third electrical path, the thirdelectrical path does not include a fourth electrical path, and thefourth electrical path is an electrical path from the drain (or thesecond terminal or the like) of the transistor to the source (or thefirst terminal or the like) of the transistor”. When the connection pathin a circuit configuration is defined by an expression similar to theabove examples, a source (or a first terminal or the like) and a drain(or a second terminal or the like) of a transistor can be distinguishedfrom each other to specify the technical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, or a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, the term “electrical connection” in thisspecification also means such a case where one conductive film hasfunctions of a plurality of components.

In this specification and the like, a structure in which a flexibleprinted circuit (FPC), a tape carrier package (TCP), or the like isattached to a substrate of a display panel, or a structure in which anintegrated circuit (IC) is directly mounted on a substrate by a chip onglass (COG) method is referred to as a display device in some cases.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention and a manufacturing method of the semiconductor devicewill be described with reference to drawings.

<Structure of Transistor 10>

FIGS. 1A to 1C are a top view and cross-sectional views which illustratea transistor 10 of one embodiment of the present invention. FIG. 1A is atop view and FIGS. 1B and 1C are cross-sectional views taken alongdashed-dotted line A1-A2 and dashed-dotted line A3-A4 in FIG. 1A,respectively. In FIG. 1A, some components are scaled up or down oromitted for simplification of the drawing. In some cases, the directionof dashed-dotted line A1-A2 is referred to as a channel lengthdirection, and the direction of dashed-dotted line A3-A4 is referred toas a channel width direction.

The transistor 10 includes a substrate 100, an insulating layer 110, anoxide insulating layer 121, an oxide semiconductor layer 122, an oxideinsulating layer 123, a low-resistance region 127, a gate insulatinglayer 150, a gate electrode layer 160, a sidewall insulating layer 176,an insulating layer 180, and conductive layers 190 and 195.

The insulating layer 110 is provided over the substrate 100.

The oxide insulating layer 121 is provided over the insulating layer110.

The oxide semiconductor layer 122 is provided over the oxide insulatinglayer 121.

The oxide insulating layer 123 is provided over the insulating layer 110and the oxide semiconductor layer 122. The oxide insulating layer 123preferably includes a region that is in contact with a side surface ofthe oxide semiconductor layer 122. Accordingly, the side surface of theoxide semiconductor layer 122 can be protected and thus the electricalcharacteristics of the transistor can be stabilized.

<Oxide Insulating Layer>

An oxide insulating layer (e.g., the oxide insulating layers 121 and123) refers to a layer which basically has an insulating property and inwhich current can flow through the interface with a semiconductor andthe vicinity thereof when a gate electric field or a drain electricfield is increased.

The low-resistance region 127 includes a region in contact with theoxide semiconductor layer 122 and the oxide insulating layer 121. Thelow-resistance region 127 may be part of the oxide semiconductor layer122 and/or the oxide insulating layer 121. The low-resistance region 127contains any one of the elements of cobalt, nickel, vanadium, andpalladium and any one of the elements of titanium, zirconium, hafnium,and tantalum. The low-resistance region 127 functions as a source and adrain.

Moreover, in the low-resistance region 127, the above material mightform an alloy with the oxide semiconductor layer 122. The low-resistanceregion 127 can have lower resistance than the oxide semiconductor layer122 by formation of the alloy.

The distance between a top surface of the low-resistance region 127 anda top surface of the substrate 100 may be the same as the distancebetween an upper end of the oxide semiconductor layer 122 and the topsurface of the substrate 100 (see FIG. 2B), or may be larger or smallerthan the distance between the upper end of the oxide semiconductor layer122 and the top surface of the substrate 100.

In the above description, the low-resistance region 127 in the oxidesemiconductor layer 122 can also be referred to as an oxide conductivelayer.

Here, in the oxide semiconductor layer 122, a channel region overlappingwith the gate electrode layer 160 is referred to as a first region, aregion overlapping with the sidewall insulating layer 176 is referred toas a second region, and a region overlapping with neither the gateelectrode layer 160 nor the sidewall insulating layer 176 is referred toas a third region. In that case, the low-resistance region 127 can beprovided in part of the second region and the third region. The secondregion may be a lightly doped drain (LDD) region when compared with thethird region. Alternatively, the low-resistance region 127 might reachthe oxide insulating layer 121.

In the above, the second region has a region with lower resistance thanthe first region, and the third region has a region with lowerresistance than the second region. The resistance can be expressed bythe resistance (e.g., sheet resistance) measurement or the impurityconcentration.

The gate insulating layer 150 is provided over the oxide insulatinglayer 123.

The gate electrode layer 160 is provided over the gate insulating layer150. Note that the gate electrode layer 160, the gate insulating layer150, the oxide insulating layer 123, and the oxide semiconductor layer122 are overlapped with each other.

The sidewall insulating layer 176 is provided over the insulating layer110 and the low-resistance region 127 and includes a region that is incontact with side surfaces of the gate insulating layer 150 and the gateelectrode layer 160.

The insulating layer 180 is provided over the insulating layer 110.

The conductive layer 190 is provided over the low-resistance region 127.The conductive layer 190 includes a region electrically connected to thelow-resistance region 127.

The conductive layer 195 is provided over the conductive layer 190.

With the above structure, parasitic capacitance between a gate and thesource or parasitic capacitance between the gate and the drain can bereduced. Thus, the cutoff frequency characteristics of the transistor 10are improved, for example, and therefore the transistor can operate athigh speed.

Furthermore, the gate, the source, and the drain of the transistor 10can be formed in a self-aligned manner; thus, alignment accuracy can beimproved and miniaturized transistors can be easily manufactured.

When seen in the channel width direction, the transistor 10 includes aregion where the gate electrode layer 160 faces the side surfaces of theoxide insulating layer 121, the oxide semiconductor layer 122, and theoxide insulating layer 123 with the gate insulating layer 150 providedtherebetween as illustrated in the cross-sectional view in FIG. 1C,which is taken along line A3-A4. That is, the oxide insulating layer121, the oxide semiconductor layer 122, and the oxide insulating layer123 are surrounded by the electric field of the gate electrode layer 160in the channel width direction when voltage is applied to the gateelectrode layer 160. The transistor structure in which a semiconductoris surrounded by the electric field of the gate electrode layer 160 isreferred to as a surrounded channel (s-channel) structure. Moreover, inthe s-channel structure, the bottom surface of the oxide semiconductorlayer 122 is located at a higher level than that of the gate electrodelayer 160.

Here, the oxide insulating layer 121, the oxide semiconductor layer 122,and the oxide insulating layer 123 are collectively referred to as anoxide. When the transistor 10 is in an on state, a channel is formed inthe entire oxide semiconductor layer 122 (bulk), so that the on-statecurrent is increased. When the transistor is in an off state, thechannel region formed in the oxide semiconductor layer 122 with a wideband gap serves as a potential barrier; as a result, the off-statecurrent can be further reduced.

<Channel Length>

Note that the channel length of a transistor refers to, for example, adistance between a source (a source region or a source electrode) and adrain (a drain region or a drain electrode) in a region where asemiconductor (or a portion where current flows in a semiconductor whenthe transistor is on) and a gate electrode overlap with each other or aregion where a channel is formed in a top view of the transistor. In onetransistor, channel lengths in all regions do not necessarily have thesame value. In other words, the channel length of one transistor is notfixed to one value in some cases. Therefore, in this specification, thechannel length is any one of values, the maximum value, the minimumvalue, or the average value in a region where a channel is formed.

<Channel Width>

Note that the channel width refers to, for example, the length of aregion where a semiconductor (or a portion where current flows in asemiconductor when a transistor is on) and a gate electrode overlap witheach other. In one transistor, channel widths in all regions do notnecessarily have the same value. In other words, the channel width ofone transistor is not fixed to one value in some cases. Therefore, inthis specification, the channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on the transistor structure, a channel width in aregion where a channel is actually formed (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is larger thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is high in some cases. In that case, an effective channelwidth obtained when a channel is actually formed is greater than anapparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example,estimation of an effective channel width from a design value requires anassumption that the shape of a semiconductor is known. Therefore,without accurate information on the shape of a semiconductor, it isdifficult to measure an effective channel width accurately.

<SCW>

Therefore, in this specification, in a top view of a transistor, anapparent channel width in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width or an apparent channel width. Alternatively, inthis specification, in the case where the term “channel width” is simplyused, it may denote an effective channel width in some cases. Note thatthe values of a channel length, a channel width, an effective channelwidth, an apparent channel width, a surrounded channel width, and thelike can be determined by obtaining and analyzing a cross-sectional TEMimage and the like.

Note that in the case where field-effect mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from the value obtained by calculation using aneffective channel width is obtained in some cases.

<Improvement of Characteristics in Miniaturization>

High integration of a semiconductor device requires miniaturization of atransistor. However, it is known that miniaturization of a transistorcauses deterioration of electrical characteristics of the transistor. Adecrease in channel width causes a reduction in on-state current.

In the transistor of one embodiment of the present invention illustratedin FIGS. 1A to 1C, for example, as described above, the oxide insulatinglayer 123 is formed so as to cover the oxide semiconductor layer 122where a channel is formed and the channel formation region and the gateinsulating layer are not in contact with each other. Accordingly,scattering of carriers at the interface between the channel formationregion and the gate insulating layer can be reduced and the on-statecurrent of the transistor can be increased.

In the transistor of one embodiment of the present invention, the gateelectrode layer 160 is formed to electrically surround the oxidesemiconductor layer 122, which is to be a channel, in the channel widthdirection; accordingly, a gate electric field is applied to the oxidesemiconductor layer 122 in the side surface direction in addition to theperpendicular direction. Furthermore, the bottom surface of the oxidesemiconductor layer 122 is located at a higher level than that of thegate electrode layer 160 and thus a gate electric field is also appliedto the bottom surface of the oxide semiconductor layer 122. In otherwords, a gate electric field is applied to the oxide semiconductor layer122 entirely, so that current flows in the whole of the oxidesemiconductor layer 122, leading to a further increase in on-statecurrent.

In the transistor of one embodiment of the present invention, the oxideinsulating layer 123 is formed over the oxide insulating layer 121 andthe oxide semiconductor layer 122, so that an interface state isunlikely to be formed. In addition, impurities do not enter the oxidesemiconductor layer 122 from above and below because the oxidesemiconductor layer 122 is positioned at the middle. Therefore, thetransistor can achieve not only the increase in the on-state current butalso stabilization of the threshold voltage and a reduction in the Svalue (subthreshold value). Thus, a drain current at a gate voltage VGof 0 V can be reduced and power consumption can be reduced. Further,since the threshold voltage of the transistor becomes stable, long-termreliability of the semiconductor device can be improved.

In the transistor of one embodiment of the present invention, the gateelectrode layer 160 is formed to electrically surround the oxidesemiconductor layer 122, which is to be a channel, in the channel widthdirection; accordingly, a gate electric field is applied to the oxidesemiconductor layer 122 in the side surface direction in addition to theperpendicular direction. That is, a gate electric field is applied tothe entire oxide semiconductor layer 122, so that the influence of adrain electric field can be reduced and a short-channel effect can besignificantly suppressed. Therefore, the transistor can have favorablecharacteristics even when miniaturized.

Alternatively, when the transistor of one embodiment of the presentinvention includes a wide band gap material as the oxide semiconductorlayer 122, which is to be the channel, the transistor can have highsource-drain breakdown voltage and stable electrical characteristics invarious temperature environments.

Although an example where a channel or the like is formed in an oxidesemiconductor layer or the like is described in this embodiment, oneembodiment of the present invention is not limited thereto. For example,depending on circumstances or conditions, a channel, the vicinity of thechannel, a source region, a drain region, or the like may be formedusing a material containing silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, an organicsemiconductor, or the like.

<Components of Transistor>

Components of a transistor of this embodiment will be described below.

<Substrate 100>

A glass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like can be used as the substrate 100. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon or silicon carbide, a compoundsemiconductor substrate of silicon germanium, a silicon on insulator(SOI) substrate, or the like can be used. Still alternatively, any ofthese substrates provided with a semiconductor element may be used. Thesubstrate 100 is not limited to a simple supporting substrate, and maybe a substrate where a device such as a transistor is formed. In thatcase, any one or more of the gate, the source, and the drain of thetransistor may be electrically connected to the device.

Alternatively, a flexible substrate may be used as the substrate 100. Asa method for providing the transistor over a flexible substrate, thereis a method in which the transistor is formed over a non-flexiblesubstrate and then the transistor is separated and transferred to thesubstrate 100 that is a flexible substrate. In that case, a separationlayer is preferably provided between the non-flexible substrate and thetransistor. As the substrate 100, a sheet, a film, or a foil containinga fiber may be used, for example. The substrate 100 may have elasticity.The substrate 100 may have a property of returning to its original shapewhen bending or pulling is stopped. Alternatively, the substrate 100 mayhave a property of not returning to its original shape. The thickness ofthe substrate 100 is, for example, greater than or equal to 5 μm andless than or equal to 700 μm, preferably greater than or equal to 10 μmand less than or equal to 500 μm and further preferably greater than orequal to 15 μm and less than or equal to 300 μm. When the substrate 100has a small thickness, the weight of the semiconductor device can bereduced. When the substrate 100 has a small thickness, even in the caseof using glass or the like, the substrate 100 might have elasticity or aproperty of returning to its original shape when bending or pulling isstopped. Therefore, an impact applied to the semiconductor device overthe substrate 100, which is caused by dropping or the like, can bereduced. That is, a durable semiconductor device can be provided.

For the substrate 100 which is a flexible substrate, metal, an alloy,resin, glass, or fiber thereof can be used, for example. The flexiblesubstrate 100 preferably has a lower coefficient of linear expansionbecause deformation due to an environment is suppressed. The flexiblesubstrate 100 is formed using, for example, a material whose coefficientof linear expansion is lower than or equal to 1×10⁻³/KI, lower than orequal to 5×10⁻⁵/K, or lower than or equal to 1×10⁻⁵/K. Examples of theresin include polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, acrylic, and polytetrafluoroethylene (PTFE).In particular, aramid is preferably used for the flexible substrate 100because of its low coefficient of linear expansion.

<Insulating Layer 110>

As the insulating layer 110, an insulating film containing one or moreof silicon (Si), nitrogen (N), oxygen (O), fluorine (F), hydrogen (H),aluminum (Al), gallium (Ga), germanium (Ge), yttrium (Y), zirconium(Zr), lanthanum (La), neodymium (Nd), hafnium (Hf), and tantalum (Ta)can be used.

The insulating layer 110 can have a function of supplying oxygen to theoxide semiconductor layer 122 as well as a function of preventingdiffusion of impurities from the substrate 100. For this reason, theinsulating layer 110 is preferably an insulating film containing oxygen,further preferably an insulating film having an oxygen content higherthan that in the stoichiometric composition. For example, the insulatinglayer 110 is a film in which the amount of released oxygen convertedinto oxygen atoms is 1.0×10¹⁹ atoms/cm³ or more in TDS analysis. Notethat the temperature of the film surface in the TDS analysis ispreferably higher than or equal to 100° C. and lower than or equal to700° C., or higher than or equal to 100° C. and lower than or equal to500° C. In the case where the substrate 100 is provided with anotherdevice as described above, the insulating layer 110 also functions as aninterlayer insulating film. In that case, the insulating layer 110 ispreferably subjected to planarization treatment such as chemicalmechanical polishing (CMP) treatment so as to have a flat surface.

When the insulating layer 110 contains fluorine, fluorine gasified fromthe insulating layer can stabilize an oxygen vacancy in the oxidesemiconductor layer 122.

<Oxide Insulating Layer 121, Oxide Semiconductor Layer 122, and OxideInsulating Layer 123>

The oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 are oxide semiconductor films containingIn or Zn and typically contain an In—Ga oxide, an In—Zn oxide, an In—Mgoxide, a Zn—Mg oxide, or an In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr,La, Ce, Mg, Hf, Fe, or Nd).

An oxide that can be used for each of the oxide insulating layer 121,the oxide semiconductor layer 122, and the oxide insulating layer 123preferably contains at least indium (In) or zinc (Zn). Alternatively,both In and Zn are preferably contained. In order to reduce variationsin electrical characteristics of the transistors including the oxide,the oxide preferably contains a stabilizer in addition to In and Zn.

As examples of a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf),aluminum (Al), zirconium (Zr), and the like can be given. As anotherexample of stabilizer, lanthanoid such as lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu),gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium(Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given.

The indium and gallium contents in the oxide insulating layer 121, theoxide semiconductor layer 122, and the oxide insulating layer 123 can becompared with each other by time-of-flight secondary ion massspectrometry (TOF-SIMS), X-ray photoelectron spectrometry (XPS), orinductively coupled plasma mass spectrometry (ICP-MS).

Since the oxide semiconductor layer 122 has an energy gap of 2 eV ormore, preferably 2.5 eV or more and further preferably 3 eV or more, theoff-state current of the transistor 10 can be low.

The thickness of the oxide semiconductor layer 122 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm and further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

The thickness of the oxide semiconductor layer 122 may be larger than,equal to, or smaller than that of the oxide insulating layer 121. If thethickness of the oxide semiconductor layer 122 is large, the on-statecurrent of the transistor can be increased. The thickness of the oxideinsulating layer 121 may be determined as appropriate as long asformation of an interface state at the interface with the oxidesemiconductor layer 122 can be suppressed. For example, the thickness ofthe oxide semiconductor layer 122 is larger than that of the oxideinsulating layer 121, preferably 2 or more times, further preferably 4or more times, and still further preferably 6 or more times, as large asthat of the oxide insulating layer 121. In the case where there is noneed to increase the on-state current of the transistor, the thicknessof the oxide insulating layer 121 may be larger than or equal to that ofthe oxide semiconductor layer 122.

In the case where the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 havedifferent compositions from one another, the interfaces thereof can beobserved with a scanning transmission electron microscope (STEM) in somecases.

The content of In in the oxide semiconductor layer 122 is preferablyhigher than those in the oxide insulating layers 121 and 123. In anoxide semiconductor layer, the s orbital of heavy metal mainlycontributes to carrier transfer, and when the proportion of In in theoxide semiconductor is increased, overlap of the s orbitals is likely tobe increased. Therefore, an oxide having a composition in which theproportion of In is higher than that of M has higher mobility than anoxide having a composition in which the proportion of In is equal to orlower than that of M. Thus, with the use of an oxide having a highindium content for the oxide semiconductor layer 122, a transistorhaving high field-effect mobility can be obtained.

In the case where the oxide semiconductor layer 122 is an In-M-Zn oxide(M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd) and a targethaving the atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is usedfor depositing the oxide semiconductor layer 122 by a sputtering method,x₂/(x₂+y₂+z₂) is preferably greater than or equal to ⅓. Furthermore,x₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to10 and further preferably greater than or equal to 1 and less than orequal to 6, and z₂/y₂ is preferably greater than or equal to ⅓ and lessthan or equal to 10 and further preferably greater than or equal to 1and less than or equal to 6. In this manner, a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) film is easily formed as theoxide semiconductor layer 122. Typical examples of the atomic ratio ofmetal elements of the target include In:M:Zn=1:1:1, 1:1:1.2, 2:1:1.5,2:1:2.3, 2:1:3, 3:1:2, 4:2:3, 4:2:4.1, 5:1:6, and 5:1:7.

When the atomic ratio of Al, Ti, Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, orNd having a function as a stabilizer is higher than that of In in eachof the oxide insulating layers 121 and 123, any of the following effectsmight be obtained.

(1) The energy gap of each of the oxide insulating layers 121 and 123 iswidened.(2) The electron affinity of each of the oxide insulating layers 121 and123 is reduced.(3) Impurities from the outside are blocked.(4) An insulating property of each of the oxide insulating layers 121and 123 is higher than that of the oxide semiconductor layer 122.(5) Oxygen vacancies are less likely to be generated in the oxideinsulating layers 121 and 123 each containing a larger amount of Al, Ti,Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, or Nd in an atomic ratio than theamount of In in an atomic ratio because Al, Ti, Ga, Y, Zr, Sn, La, Ce,Mg, Hf, Fe, and Nd are metal elements that can be strongly bonded tooxygen.

By providing an oxide film in which oxygen vacancies are less likely tobe generated than in the oxide semiconductor layer 122, over and underand in contact with the oxide semiconductor layer 122, oxygen vacanciesin the oxide semiconductor layer 122 can be reduced. Further, since theoxide semiconductor layer 122 is in contact with the oxide insulatinglayers 121 and 123 containing one or more metal elements forming theoxide semiconductor layer 122, the density of interface states at theinterface between the oxide insulating layer 121 and the oxidesemiconductor layer 122 and at the interface between the oxidesemiconductor layer 122 and the oxide insulating layer 123 is extremelylow. Therefore, after oxygen is added to the oxide insulating layer 121,the oxide insulating layer 123, the gate insulating layer 150, theinsulating layer 110, or the insulating layer 180, the oxygen istransferred through the oxide insulating layers 121 and 123 to the oxidesemiconductor layer 122 by heat treatment; however, the oxygen is hardlytrapped by the interface states at this time, and the oxygen in theoxide insulating layer 121 or 123 can be efficiently transferred to theoxide semiconductor layer 122. Accordingly, oxygen vacancies in theoxide semiconductor layer 122 can be reduced. Since oxygen is added tothe oxide insulating layers 121 and 123, oxygen vacancies in the oxideinsulating layers 121 and 123 can be reduced. In other words, thedensity of localized states of at least the oxide semiconductor layer122 can be reduced.

In addition, when the oxide semiconductor layer 122 is in contact withan insulating film including a different constituent element (e.g., agate insulating layer including a silicon oxide film), an interfacestate is sometimes formed and the interface state forms a channel. Atthis time, a second transistor having a different threshold voltageappears, so that an apparent threshold voltage of the transistor isvaried. However, since the oxide insulating layers 121 and 123containing one or more kinds of metal elements forming the oxidesemiconductor layer 122 are in contact with the oxide semiconductorlayer 122, an interface state is not easily formed at the interfacebetween the oxide insulating layer 121 and the oxide semiconductor layer122 and at the interface between the oxide insulating layer 123 and theoxide semiconductor layer 122.

In this embodiment, the amount of oxygen vacancies in the oxidesemiconductor layer 122, and further the amount of oxygen vacancies inthe oxide insulating layers 121 and 123 in contact with the oxidesemiconductor layer 122 can be reduced; thus, the density of localizedstates of the oxide semiconductor layer 122 can be reduced. As a result,the transistor 10 in this embodiment has small variations in thresholdvoltage and high reliability. Further, the transistor 10 of thisembodiment has excellent electrical characteristics.

In the case where the gate insulating layer 150 and the oxidesemiconductor layer 122 are in contact with each other and a channel isformed at the interface therebetween, interface scattering occurs at theinterface and the field-effect mobility of the transistor is decreased.However, since the oxide insulating layers 121 and 123 containing one ormore kinds of metal elements forming the oxide semiconductor layer 122are provided in contact with the oxide semiconductor layer 122,scattering of carriers does not easily occur at the interface betweenthe oxide semiconductor layer 122 and the oxide insulating layer 121 andat the interface between the oxide semiconductor layer 122 and the oxideinsulating layer 123, and thus the field-effect mobility of thetransistor can be increased.

Each of the oxide insulating layers 121 and 123 is typically an In—Gaoxide, an In—Zn oxide, an In—Mg oxide, a Ga—Zn oxide, a Zn—Mg oxide, oran In-M-Zn oxide (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, orNd), and has the energy level at the conduction band minimum that iscloser to a vacuum level than the energy level at the conduction bandminimum of the oxide semiconductor layer 122 is. Typically, a differencebetween the energy level at the conduction band minimum of the oxidesemiconductor layer 122 and the energy level at the conduction bandminimum of each of the oxide insulating layers 121 and 123 is greaterthan or equal to 0.05 eV, greater than or equal to 0.07 eV, greater thanor equal to 0.1 eV, or greater than or equal to 0.2 eV and also lessthan or equal to 2 eV, less than or equal to 1 eV, less than or equal to0.5 eV, or less than or equal to 0.4 eV. That is, the difference betweenthe electron affinity of the oxide semiconductor layer 122 and theelectron affinity of each of the oxide insulating layers 121 and 123 isgreater than or equal to 0.05 eV, greater than or equal to 0.07 eV,greater than or equal to 0.1 eV, or greater than or equal to 0.2 eV andalso less than or equal to 2 eV, less than or equal to 1 eV, less thanor equal to 0.5 eV, or less than or equal to 0.4 eV. Note that theelectron affinity refers to a difference between the vacuum level andthe energy level at the conduction band minimum.

In the case where the oxide insulating layers 121 and 123 are In-M-Znoxides (M is Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd), theoxide insulating layers 121 and 123 have a higher atomic ratio of M (Al,Ti, Ga, Y, Zr, Sn, La, Ce, Mg, Hf, Fe, or Nd) than the oxidesemiconductor layer 122, and the element represented by M is morestrongly bonded to oxygen than indium is; thus, generation of oxygenvacancies in the oxide insulating layers 121 and 123 can be suppressed.That is, the oxide insulating layers 121 and 123 are oxide semiconductorfilms in which oxygen vacancies are less likely to be generated than inthe oxide semiconductor layer 122.

In the case where the oxide insulating layer 121 is an In-M-Zn oxide (Mis Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd) and a target havingthe atomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used fordepositing the oxide insulating layer 121, x₁/y₁ is preferably less thanz₁/y₁, and z₁/y₁ is preferably greater than or equal to 0.1 and lessthan or equal to 6 and further preferably greater than or equal to 0.2and less than or equal to 3.

In the case where the oxide insulating layer 123 is an In-M-Zn oxide (Mis Al, Ti, Ga, Y, Sn, Zr, La, Ce, Mg, Hf, Fe, or Nd) and a target havingthe atomic ratio of metal elements of In:M:Zn=x₃:y₃:z₃ is used fordepositing the oxide insulating layers 121 and 123 by a sputteringmethod, x₃/y₃ is preferably less than x₂/y₂, and z₃/y₃ is preferablygreater than or equal to ⅓ and less than or equal to 6 and furtherpreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₃/y₃ is greater than or equal to 1 and less than or equal to6, CAAC-OS films are easily formed as the oxide insulating layer 121 andthe oxide insulating layer 123.

Typical examples of the atomic ratio between the metal elements in atarget used for depositing the oxide insulating layer 121 and the oxideinsulating layer 123 by a sputtering method include In:M:Zn=1:3:2,In:M:Zn=1:3:4, In:M:Zn=1:3:6, In:M:Zn=1:3:8, In:M:Zn=1:4:4,In:M:Zn=1:4:5, In:M:Zn=1:4:6, In:M:Zn=1:4:7, In:M:Zn=1:4:8,In:M:Zn=1:5:5, In:M:Zn=1:5:6, In:M:Zn=1:5:7, In:M:Zn=1:5:8,In:M:Zn=1:6:8, In:M:Zn=1:6:4, In:M:Zn=1:9:6, In:M:Zn=1:10:1, andGa:Zn=10:1. The atomic ratio is not limited to the above, and the atomicratio may be appropriately set in accordance with needed semiconductorcharacteristics.

Since the oxide insulating layers 121 and 123 have higher insulatingproperties than the oxide semiconductor layer 122, they each can have afunction of a gate insulating layer.

Alternatively, the oxide insulating layer 123 can be metal oxide, suchas aluminum oxide (AlO_(x)), gallium oxide (GaO_(x)), hafnium oxide(HfO_(x)), silicon oxide (SiO_(x)), germanium oxide (GeO_(x)), orzirconia oxide (ZrO_(x)); or the metal oxide may be provided over theoxide insulating layer 123.

The thickness of the oxide insulating layer 123 may be determined asappropriate as long as formation of an interface state at the interfacewith the oxide semiconductor layer 122 is inhibited. For example, thethickness of the oxide insulating layer 123 may be set smaller than orequal to that of the oxide insulating layer 121. If the thickness of theoxide insulating layer 123 is large, it might become difficult for theelectric field from the gate electrode layer 160 to reach the oxidesemiconductor layer 122. For this reason, the thickness of the oxideinsulating layer 123 is preferably small. For example, the thickness ofthe oxide insulating layer 123 is smaller than that of the oxidesemiconductor layer 122. Note that the thickness of the oxide insulatinglayer 123 is not limited to the above, and may be determined asappropriate in accordance with the driving voltage of the transistor inconsideration of the withstand voltage of the gate insulating layer 150.

For example, the thickness of the oxide insulating layer 123 ispreferably greater than or equal to 1 nm and less than or equal to 20 nmor greater than or equal to 3 nm and less than or equal to 10 nm.

In each of the oxide insulating layer 121, the oxide semiconductor layer122, and the oxide insulating layer 123, the proportion of each atom inthe above-described atomic ratio varies within a range of ±40% as anerror in some cases.

For example, when an oxide semiconductor film to be the oxidesemiconductor layer 122 is deposited by a sputtering method using atarget in which the atomic ratio of the metal elements isIn:Ga:Zn=1:1:1, the atomic ratio of the metal elements of the oxidesemiconductor film to be the oxide semiconductor layer 122 isapproximately In:Ga:Zn=1:1:0.6, which means that the atomic ratio ofzinc is not changed or reduced in some cases. Therefore, the atomicratio described in this specification includes the atomic ratio invicinity thereof.

<Hydrogen Concentration>

Hydrogen contained in the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 reacts withoxygen bonded to a metal atom to be water, and in addition, an oxygenvacancy is formed in a lattice from which oxygen is released (or aportion from which oxygen is released). An electron serving as a carriercan be generated due to entry of hydrogen into the oxygen vacancy or dueto bonding of part of hydrogen to oxygen bonded to a metal atom. Thus, atransistor including an oxide semiconductor layer which containshydrogen is likely to be normally on.

Accordingly, it is preferable that hydrogen be reduced as much aspossible as well as the oxygen vacancies in the oxide insulating layer121, the oxide semiconductor layer 122, and the oxide insulating layer123 and at the interfaces between the oxide insulating layer 121, theoxide semiconductor layer 122, and the oxide insulating layer 123. Theconcentrations of hydrogen in the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 and at theinterfaces between the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123, which areobtained by secondary ion mass spectrometry (SIMS), are desirably higherthan or equal to 1×10¹⁶ atoms/cm³ and lower than or equal to 2×10²⁰atoms/cm³, preferably higher than or equal to 1×10¹⁶ atoms/cm³ and lowerthan or equal to 5×10¹⁹ atoms/cm³, further preferably higher than orequal to 1×10¹⁶ atoms/cm³ and lower than or equal to 1×10¹⁹ atoms/cm³,and still further preferably higher than or equal to 1×10¹⁶ atoms/cm³and lower than or equal to 5×10¹⁸ atoms/cm³. As a result, the transistor10 can have positive threshold voltage (normally-off characteristics).

<Concentrations of Carbon and Silicon>

When silicon and carbon, which are elements belonging to Group 14, arecontained in the oxide insulating layer 121, the oxide semiconductorlayer 122, and the oxide insulating layer 123 and at the interfacesbetween the oxide insulating layer 121, the oxide semiconductor layer122, and the oxide insulating layer 123, oxygen vacancies are increasedand an n-type region is formed in the oxide insulating layer 121, theoxide semiconductor layer 122, and the oxide insulating layer 123 insome cases. It is therefore preferable to reduce the concentrations ofsilicon and carbon in the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 and at theinterfaces between the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123. Theconcentrations of silicon and carbon in the oxide insulating layer 121,the oxide semiconductor layer 122, and the oxide insulating layer 123and at the interfaces between the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123, which areobtained by SIMS, are desirably higher than or equal to 1×10¹⁶ atoms/cm³and lower than or equal to 1×10¹⁹ atoms/cm³, preferably higher than orequal to 1×10¹⁶ atoms/cm³ and lower than or equal to 5×10¹⁸ atoms/cm³and further preferably higher than or equal to 1×10¹⁶ atoms/cm³ andlower than or equal to 2×10¹⁸ atoms/cm³. As a result, the transistor 10can have positive threshold voltage.

<Concentration of Alkali Metal>

Alkali metal and alkaline earth metal can generate carriers when bondedto an oxide semiconductor, which can increase the off-state current ofthe transistor. It is thus preferable to reduce the concentrations ofalkali metal and alkaline earth metal in the oxide insulating layer 121,the oxide semiconductor layer 122, and the oxide insulating layer 123and at the interfaces between the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123. Forexample, the concentrations of alkali metal and alkaline earth metal inthe oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 and at the interfaces between the oxideinsulating layer 121, the oxide semiconductor layer 122, and the oxideinsulating layer 123, which are obtained by SIMS, are desirably lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. As a result, the transistor 10 can have positivethreshold voltage.

<Concentration of Nitrogen>

When nitrogen is contained in the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 and at theinterfaces between the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123, an electronserving as a carrier is generated and accordingly carrier density isincreased, so that n-type regions are formed in some cases. Thus, atransistor including an oxide semiconductor layer which containsnitrogen is likely to be normally on. Thus, it is preferable thatnitrogen be reduced as much as possible in the oxide insulating layer121, the oxide semiconductor layer 122, and the oxide insulating layer123 and at the interfaces between the oxide insulating layer 121, theoxide semiconductor layer 122, and the oxide insulating layer 123. Forexample, the concentrations of nitrogen in the oxide insulating layer121, the oxide semiconductor layer 122, and the oxide insulating layer123 and at the interfaces between the oxide insulating layer 121, theoxide semiconductor layer 122, and the oxide insulating layer 123, whichare obtained by SIMS, are preferably higher than or equal to 1×10¹⁵atoms/cm³ and lower than or equal to 5×10¹⁹ atoms/cm³, furtherpreferably higher than or equal to 1×10¹⁵ atoms/cm³ and lower than orequal to 5×10¹⁸ atoms/cm³, still further preferably higher than or equalto 1×10¹⁵ atoms/cm³ and lower than or equal to 1×10¹⁸ atoms/cm³, and yetstill further preferably higher than or equal to 1×10¹⁵ atoms/cm³ andlower than or equal to 5×10¹⁷ atoms/cm³. As a result, the transistor 10can have positive threshold voltage.

However, in the case where excess zinc exists in the oxide semiconductorlayer 122, the concentrations of nitrogen are not limited to the aboverange. Excess zinc might cause oxygen vacancies in the oxidesemiconductor layer 122, and when the oxide semiconductor layer 122containing excess zinc also contains nitrogen at 0.001 atomic % to 3atomic %, the oxygen vacancies caused by the excess zinc can beinactivated in some cases. Therefore, the nitrogen can reduce variationsin transistor characteristics and can improve the reliability.

<Carrier Density>

The carrier densities of the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 can belowered by reduction in impurities in the oxide insulating layer 121,the oxide semiconductor layer 122, and the oxide insulating layer 123.The carrier densities of the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 are lessthan or equal to 1×10¹⁵/cm³, preferably less than or equal to1×10¹³/cm³, further preferably less than 8×10¹¹/cm³, still furtherpreferably less than 1×10¹¹/cm³, and yet still further preferably lessthan 1×10¹⁰/cm³, and is greater than or equal to 1×10⁻⁹/cm³.

As described above, when an oxide having a low impurity concentrationand a low density of defect states is used as each of the oxideinsulating layer 121, the oxide semiconductor layer 122, and the oxideinsulating layer 123, the transistor can have more excellent electricalcharacteristics. Here, the state in which the impurity concentration islow and the density of defect states is low (the amount of oxygenvacancies is small) is described as “highly purified intrinsic” or“substantially highly purified intrinsic”. A highly purified intrinsicor substantially highly purified intrinsic oxide has few carriergeneration sources, and thus can have a low carrier density in somecases. Thus, a transistor including the oxide in which a channel regionis formed is likely to have positive threshold voltage. A highlypurified intrinsic or substantially highly purified intrinsic oxide hasa low density of defect states and accordingly has a low density of trapstates in some cases. Further, a highly purified intrinsic orsubstantially highly purified intrinsic oxide has an extremely lowoff-state current; the off-state current can be lower than or equal tothe measurement limit of a semiconductor parameter analyzer, i.e., lowerthan or equal to 1×10⁻¹³ A, at a voltage between a source electrode anda drain electrode (drain voltage) of from 1 V to 10 V. Thus, thetransistor whose channel region is formed in the oxide can have a smallvariation in electrical characteristics and high reliability.

A transistor in which a highly purified oxide is used for a channelformation region exhibits extremely low off-state current. For example,in the case where the voltage between the source and the drain is set toapproximately 0.1 V, 5 V, or 10 V, the off-state current standardized onthe channel width of the transistor can be as low as severalyoctoamperes per micrometer to several zeptoamperes per micrometer.

The oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 may have a non-single crystal structure,for example. The non-single crystal structure includes a CAAC-OS whichis described later, a polycrystalline structure, a microcrystallinestructure, or an amorphous structure, for example. Among the non-singlecrystal structures, the amorphous structure has the highest density ofdefect states, whereas the CAAC-OS has the lowest density of defectstates.

The oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 may have a microcrystalline structure,for example. In some cases, the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 which havethe microcrystalline structure each include a microcrystal with a sizegreater than or equal to 1 nm and less than 10 nm, for example.Alternatively, the oxide films and the oxide semiconductor film whichhave the microcrystalline structure each have a mixed phase structurewhere crystal parts (each of which is greater than or equal to 1 nm andless than 10 nm in size) are distributed in an amorphous phase, forexample.

The oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 may have an amorphous structure, forexample. The oxide insulating layer 121, the oxide semiconductor layer122, and the oxide insulating layer 123 which have the amorphousstructure each have disordered atomic arrangement and no crystallinecomponent, for example. Alternatively, an oxide film having an amorphousstructure has, for example, an absolutely amorphous structure and nocrystal part.

The oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 may each be a mixed film includingregions having two or more of the following structures: a CAAC-OS, amicrocrystalline structure, and an amorphous structure. The mixed film,for example, has a single-layer structure including a region having anamorphous structure, a region having a microcrystalline structure, and aregion of a CAAC-OS. Alternatively, the mixed film may have astacked-layer structure including a region having an amorphousstructure, a region having a microcrystalline structure, and a region ofa CAAC-OS, for example.

Note that the oxide insulating layer 121, the oxide semiconductor layer122, and the oxide insulating layer 123 may have a single-crystalstructure, for example.

The oxide insulating layers 121 and 123 function as barrier films thatprevent constituent elements of the insulating layer 110 and the gateinsulating layer 150 from entering the oxide semiconductor layer 122 andforming an impurity state.

For example, in the case of using a silicon-containing insulating filmas the insulating layer 110 or the gate insulating layer 150, silicon inthe gate insulating layer 150 or carbon which might be contained in theinsulating layer 110 or the gate insulating layer 150 enters the oxideinsulating layer 121 or 123 to a depth of several nanometers from theinterface in some cases. An impurity, such as silicon or carbon,entering the oxide semiconductor layer 122 forms an impurity state. Theimpurity state serves as a donor to generate an electron; thus, theoxide semiconductor layer 122 might become n-type.

However, when each thickness of the oxide insulating layers 121 and 123is larger than several nanometers, the impurity such as silicon orcarbon does not reach the oxide semiconductor layer 122, so that theinfluence of impurity states is reduced.

Thus, providing the oxide insulating layers 121 and 123 makes itpossible to reduce variations in electrical characteristics of thetransistor, such as threshold voltage.

Accordingly, with the stacked-layer structure including the oxideinsulating layer 121, the oxide semiconductor layer 122, and the oxideinsulating layer 123, a channel can be formed in the oxide semiconductorlayer 122; thus, the transistor can have a high field-effect mobilityand stable electrical characteristics.

Note that the oxide semiconductor layer does not necessarily have athree-layer structure including the oxide insulating layers and can havea single layer, two layers, four layers, or five or more layers. In thecase of a single layer, a layer corresponding to the oxide semiconductorlayer 122, which is described in this embodiment, can be used.

<Band Diagram>

Here, a band diagram of the transistor of one embodiment of the presentinvention will be described with reference to FIGS. 3A and 3B. For easyunderstanding, the band diagram in FIG. 3B is illustrated with theenergy levels (Ec) at the conduction band minimum and the energy levels(Ev) at the valence band maximum of the insulating layer 110, the oxideinsulating layer 121, the oxide semiconductor layer 122, the oxideinsulating layer 123, and the gate insulating layer 150.

As illustrated in FIGS. 3A and 3B, the energy level at the conductionband minimum changes continuously within the oxide insulating layer 121,the oxide semiconductor layer 122, and the oxide insulating layer 123.This can be understood also from the fact that the constituent elementsare common among the oxide insulating layer 121, the oxide semiconductorlayer 122, and the oxide insulating layer 123 and oxygen is easilydiffused among them. Thus, the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123 have acontinuous physical property although they are a stack of films havingdifferent compositions.

Oxide semiconductor films, which contain the same main components andare stacked, are not simply stacked but formed to have continuousjunction (here, particularly a U-shaped (U shape) well structure wherethe energy level at the conduction band minimum is continuously changedbetween the layers). In other words, a stacked-layer structure is formedsuch that there exist no impurities which form a defect state such as atrap center or a recombination center at each interface. If impuritiesare mixed between the stacked layers in the multilayer film, thecontinuity of the energy band is lost and carriers disappear by a trapor recombination at the interface.

Although Ec of the oxide insulating layer 121 and that of the oxideinsulating layer 123 are equal to each other, they may be different.

As illustrated in FIG. 3B, the oxide semiconductor layer 122 serves as awell and a channel of the transistor 10 is formed in the oxidesemiconductor layer 122. Note that a channel having a U-shaped wellstructure in which the energy level at the conduction band minimumcontinuously changes like the one formed in the oxide semiconductorlayer 122, can also be referred to as a buried channel.

Note that trap states due to impurities or defects can be formed in thevicinity of the interface between an insulating film such as a siliconoxide film and each of the oxide insulating layers 121 and 123. Theoxide semiconductor layer 122 can be distanced away from the trap statesowing to existence of the oxide insulating layers 121 and 123. However,when the energy difference between Ec of the oxide insulating layer 121or 123 and Ec of the oxide semiconductor layer 122 is small, an electronin the oxide semiconductor layer 122 might reach the trap state bypassing over the energy difference. When electrons to be negative chargeare captured by the trap states, a negative fixed charge is generated atthe interface with the insulating film, whereby the threshold voltage ofthe transistor is shifted in the positive direction. In addition, a trapis not fixed and electrical characteristics can be changed in along-time preservation test of the transistor.

Thus, to reduce a change in the threshold voltage of the transistor, theenergy difference between the Ec of each of the oxide insulating layers121 and 123 and the Ec of the oxide semiconductor layer 122 isnecessary. The energy difference is preferably greater than or equal to0.1 eV and further preferably greater than or equal to 0.2 eV.

The oxide insulating layer 121, the oxide semiconductor layer 122, andthe oxide insulating layer 123 preferably include a crystal part. Inparticular, when a crystal in which c-axes are aligned is used, thetransistor can have stable electrical characteristics.

In the band diagram illustrated in FIG. 3B, an In—Ga oxide (e.g., anIn—Ga oxide with an atomic ratio of In:Ga=7:93), gallium oxide, or thelike may be provided between the oxide semiconductor layer 122 and thegate insulating layer 150 without providing the oxide insulating layer123. Alternatively, an In—Ga oxide, gallium oxide, or the like may beprovided between the oxide insulating layer 123 and the gate insulatinglayer 150.

As the oxide semiconductor layer 122, an oxide having an electronaffinity higher than those of the oxide insulating layers 121 and 123 isused. The oxide which can be used for the oxide semiconductor layer 122has, for example, an electron affinity higher than that of each of theoxide insulating layers 121 and 123 by 0.07 eV or higher and 1.3 eV orlower, preferably 0.1 eV or higher and 0.7 eV or lower, and furtherpreferably 0.2 eV or higher and 0.4 eV or lower.

Since the transistor described in this embodiment includes the oxideinsulating layers 121 and 123 that each include one or more kinds ofmetal elements included in the oxide semiconductor layer 122, aninterface state is less likely to be formed at the interface between theoxide insulating layer 121 and the oxide semiconductor layer 122 and theinterface between the oxide insulating layer 123 and the oxidesemiconductor layer 122. Thus, providing the oxide insulating layers 121and 123 makes it possible to reduce variations or changes in electricalcharacteristics of the transistor, such as threshold voltage.

<Gate Insulating Layer 150>

The gate insulating layer 150 can contain oxygen (O), nitrogen (N),fluorine (F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga),germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium(Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. Forexample, the gate insulating layer 150 can contain one or more ofaluminum oxide (AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide(SiO_(x)), silicon oxynitride (SiO_(x)N_(y)), silicon nitride oxide(SiN_(x)O_(y)), silicon nitride (SiN_(x)), gallium oxide (GaO_(Y)),germanium oxide (GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide(ZrO_(x)), lanthanum oxide (LaO_(x)), neodymium oxide (NdO_(x)), hafniumoxide (HfO_(x)), and tantalum oxide (TaO_(x)). The gate insulating layer150 may be a stack of any of the above materials. The gate insulatinglayer 150 may contain lanthanum (La), nitrogen, zirconium (Zr), or thelike as an impurity.

The gate insulating layer 150 preferably contains a large amount ofoxygen. Oxygen contained in the gate insulating layer 150 reaches theoxide semiconductor layer 122 through the oxide insulating layer 123 byheat treatment. Accordingly, oxygen vacancies (Vo) in the oxidesemiconductor layer 122 can be reduced.

An example of a stacked-layer structure of the gate insulating layer 150will be described. The gate insulating layer 150 contains, for example,oxygen, nitrogen, silicon, or hafnium. Specifically, the gate insulatinglayer 150 preferably contains hafnium oxide, and silicon oxide orsilicon oxynitride.

Hafnium oxide has higher dielectric constant than silicon oxide andsilicon oxynitride. Therefore, the gate insulating layer 150 usinghafnium oxide can have larger thickness than the gate insulating layer150 using silicon oxide, so that leakage current due to tunnel currentcan be reduced. That is, it is possible to provide a transistor with alow off-state current. Moreover, hafnium oxide with a crystallinestructure has higher dielectric constant than hafnium oxide with anamorphous structure. Therefore, it is preferable to use hafnium oxidewith a crystalline structure in order to provide a transistor with a lowoff-state current. Examples of the crystalline structure include amonoclinic crystal structure and a cubic crystal structure. Note thatone embodiment of the present invention is not limited to the aboveexamples.

A surface over which the hafnium oxide with a crystalline structure isformed might have interface states due to defects. The interface stateserves as a trap center in some cases. Therefore, when hafnium oxide isprovided near a channel region of a transistor, the electricalcharacteristics of the transistor might deteriorate because of theinterface state. In order to reduce the adverse effect of the interfacestate, in some cases, it is preferable to separate the channel region ofthe transistor and the hafnium oxide from each other by providinganother film therebetween. The film has a buffer function. The filmhaving a buffer function may be included in the gate insulating layer150 or included in an oxide semiconductor film. That is, the film havinga buffer function can be formed using silicon oxide, silicon oxynitride,an oxide semiconductor, or the like. Note that the film having a bufferfunction is formed using, for example, a semiconductor or an insulatorhaving a larger energy gap than a semiconductor to be the channelregion. Alternatively, the film having a buffer function is formedusing, for example, a semiconductor or an insulator having lowerelectron affinity than a semiconductor to be the channel region. Furtheralternatively, the film having a buffer function is formed using, forexample, a semiconductor or an insulator having higher ionization energythan a semiconductor to be the channel region.

In some cases, the threshold voltage of a transistor can be controlledby trapping an electric charge in an interface state (trap center) atthe surface over which the hafnium oxide with a crystalline structure isformed. In order to make the electric charge exist stably, for example,an insulator having a larger energy gap than hafnium oxide may beprovided between the channel region and the hafnium oxide.Alternatively, a semiconductor or an insulator having lower electronaffinity than hafnium oxide may be provided. The film having a bufferfunction may be formed using a semiconductor or an insulator havinghigher ionization energy than hafnium oxide. With the use of such aninsulator, an electric charge trapped in the interface state is lesslikely to be released; accordingly, the electric charge can be held fora long period of time.

Examples of such an insulator include silicon oxide and siliconoxynitride. In order to make the interface state in the gate insulatinglayer 150 trap an electric charge, an electron is transferred from theoxide semiconductor layer 122 toward the gate electrode layer 160. As aspecific example, the potential of the gate electrode layer 160 is kepthigher than the potential of the source or the drain at hightemperatures (e.g., a temperature higher than or equal to 125° C. andlower than or equal to 450° C., typically higher than or equal to 150°C. and lower than or equal to 300° C.) for one second or longer,typically for one minute or longer.

The threshold voltage of a transistor in which a predetermined amount ofelectrons are trapped in interface states in the gate insulating layer150 or the like shifts in the positive direction. The amount ofelectrons to be trapped (the amount of change in threshold voltage) canbe controlled by adjusting a voltage of the gate electrode layer 160 ortime in which the voltage is applied. Note that a location in which anelectric charge is trapped is not necessarily limited to the inside ofthe gate insulating layer 150 as long as an electric charge can betrapped therein. A stacked film having a similar structure may be usedas a different insulating layer.

<Gate Electrode Layer 160>

The gate electrode layer 160 can be formed using aluminum (Al), titanium(Ti), chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), yttrium (Y),zirconium (Zr), molybdenum (Mo), ruthenium (Ru), silver (Ag), tantalum(Ta), tungsten (W), or silicon (Si), for example. The gate electrodelayer 160 may have a stacked-layer structure. When a stacked-layerstructure is employed, the above materials may be combined with amaterial containing nitrogen, such as a nitride of any of the abovematerials.

<Sidewall Insulating Layer 176>

The sidewall insulating layer 176 can be formed using a material similarto that of the gate insulating layer 150.

<Insulating Layer 180>

The insulating layer 180 can be formed using a material similar to thatof the gate insulating layer 150.

The insulating layer 180 may be a stack. The insulating layer 180preferably contains oxygen more than that in the stoichiometriccomposition. Oxygen released from the insulating layer 180 can bediffused into the channel formation region in the oxide semiconductorlayer 122 through the gate insulating layer 150, so that oxygenvacancies formed in the channel formation region can be filled with theoxygen. In this manner, stable electrical characteristics of thetransistor can be achieved.

<Conductive Layer 190>

The conductive layer 190 can be formed using a material similar to thatof the gate electrode layer 160.

<Conductive Layer 195>

The conductive layer 195 can be formed using a material similar to thatof the gate electrode layer 160.

<Manufacturing Method of Transistor>

Next, a manufacturing method of a semiconductor device of thisembodiment will be described with reference to FIGS. 4A to 4D, FIGS. 5Aand 5B, FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS. 9A to 9C,FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13A to 13C,FIGS. 14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS. 17A to 17C,and FIGS. 18A to 18C. Note that the same parts as those in the abovetransistor structure are not described here. The direction of A1-A2 andthat of A3-A4 in FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A to 8C, FIGS.9A to 9C, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS.13A to 13C, FIGS. 14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS.17A to 17C, and FIGS. 18A to 18C are respectively referred to as achannel length direction in FIGS. 1A and 1B and a channel widthdirection in FIGS. 1A and 1C in some cases.

In this embodiment, the layers included in the transistor (i.e., theinsulating layer, the oxide semiconductor layer, the conductive layer,and the like) can be deposited by any of a sputtering method, a chemicalvapor deposition (CVD) method, a vacuum evaporation method, and a pulsedlaser deposition (PLD) method. Alternatively, a coating method or aprinting method can be used. Although the sputtering method and aplasma-CVD method are typical examples of the film formation method, athermal CVD method may be used. As the thermal CVD method, a metalorganic chemical vapor deposition (MOCVD) method or an atomic layerdeposition (ALD) method may be used, for example. As the sputteringmethod, a combination of a long throw sputtering method and a collimatedsputtering method is employed, whereby the embeddability can beimproved.

<Thermal CVD Method>

A thermal CVD method has an advantage that no defect due to plasmadamage is generated because it does not utilize plasma for forming afilm.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to a chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is caused in the vicinity of thesubstrate or over the substrate.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in theembodiment can be deposited by a thermal CVD method such as a MOCVDmethod or an ALD method. For example, in the case where an In—Ga—Zn—Ofilm is formed, trimethylindium, trimethylgallium, and dimethylzinc canbe used. Note that the chemical formula of trimethylindium is In(CH₃)₃.The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemicalformula of dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium, and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

<Ald Method>

In a conventional deposition apparatus utilizing a CVD method, one ormore kinds of source gases (precursors) for reaction are supplied to achamber at the same time at the time of deposition. In a depositionapparatus utilizing an ALD method, precursors for reaction aresequentially introduced into a chamber, and then the sequence of the gasintroduction is repeated. For example, two or more kinds of precursorsare sequentially supplied to the chamber by switching respectiveswitching valves (also referred to as high-speed valves). For example, afirst precursor is introduced, an inert gas (e.g., argon or nitrogen) orthe like is introduced after the introduction of the first precursor sothat the plural kinds of precursors are not mixed, and then a secondprecursor is introduced. Alternatively, the first precursor may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second precursor may be introduced.

FIGS. 4A to 4D illustrate a deposition process by an ALD method. Firstprecursors 601 are adsorbed onto a substrate surface (see FIG. 4A),whereby a first monolayer is formed (see FIG. 4B). At this time, metalatoms and the like included in the precursors can be bonded to hydroxylgroups that exist at the substrate surface. The metal atoms may bebonded to alkyl groups such as methyl groups or ethyl groups. The firstmonolayer reacts with second precursors 602 introduced after the firstprecursors 601 are evacuated (see FIG. 4C), whereby a second monolayeris stacked over the first monolayer. Thus, a thin film is formed (seeFIG. 4D). For example, in the case where an oxidizer is included in thesecond precursors, the oxidizer chemically reacts with metal atomsincluded in the first precursors or an alkyl group bonded to metalatoms, whereby an oxide film can be formed.

An ALD method is a deposition method based on a surface chemicalreaction, by which precursors are adsorbed onto a surface and adsorbingis stopped by a self-terminating mechanism, whereby a layer isdeposited. For example, precursors such as trimethylaluminum react withhydroxyl groups (OH groups) that exist at the surface. At this time,only a surface reaction due to heat occurs; therefore, the precursorscome into contact with the surface and metal atoms or the like in theprecursors can be adsorbed onto the surface through thermal energy. Theprecursors have characteristics of, for example, having a high vaporpressure, being thermally stable before being deposited and notdissolving, and being chemically adsorbed onto a substrate at a highspeed. Since the precursors are introduced in a state of a gas, when theprecursors, which are alternately introduced, have enough time to bediffused, a film can be deposited with good coverage even onto a regionhaving unevenness with a high aspect ratio.

In an ALD method, the sequence of the gas introduction is repeated aplurality of times until a desired thickness is obtained, whereby a thinfilm with excellent step coverage can be formed. The thickness of thethin film can be adjusted by the number of repetition times of thesequence of the gas introduction; therefore, an ALD method makes itpossible to accurately adjust a thickness. The deposition rate can beincreased and the impurity concentration in the film can be furtherreduced by improving the evacuation capability.

ALD methods include an ALD method using heating (thermal ALD method) andan ALD method using plasma (plasma ALD method). In the thermal ALDmethod, precursors react using thermal energy, and in the plasma ALDmethod, precursors react in a state of a radical.

When a film is deposited by an ALD method, an extremely thin film can bedeposited with high accuracy. In addition, even the coverage of anuneven surface with the film can be improved by forming a film with anALD method.

<Plasma ALD>

Alternatively, when the plasma ALD method is employed, the film can bedeposited at a lower temperature than when the thermal ALD method isemployed. With the plasma ALD method, for example, the film can bedeposited without decreasing the deposition rate even at 100° C. orlower. Moreover, in the plasma ALD method, nitrogen radicals can beformed by plasma; thus, a nitride film as well as an oxide film can beformed.

In addition, oxidizability of an oxidizer can be enhanced by the plasmaALD method. Thus, precursors remaining in a plasma ALD film or organiccomponents released from precursors can be reduced. In addition, carbon,chlorine, hydrogen, and the like in the film can be reduced. Therefore,a film with low impurity concentration can be formed.

In the case of using the plasma ALD method, radical species aregenerated and plasma can be generated from a place apart from thesubstrate like inductively coupled plasma (ICP) or the like, so thatplasma damage to the substrate or a film on which the protective film isformed can be inhibited.

As described above, with the plasma ALD method, the film can bedeposited in the state where the process temperature can be lowered andthe coverage of the surface can be increased as compared with otherdeposition methods. Thus, entry of water and hydrogen from the outsidecan be inhibited, leading to an improvement of the reliability ofcharacteristics of the transistor.

<ALD Apparatus>

FIG. 5A illustrates an example of a deposition apparatus utilizing anALD method. The deposition apparatus utilizing an ALD method includes adeposition chamber (chamber 1701), source material supply portions 1711a and 1711 b, high-speed valves 1712 a and 1712 b which are flow ratecontrollers, source material introduction ports 1713 a and 1713 b, asource material exhaust port 1714, and an evacuation unit 1715. Thesource material introduction ports 1713 a and 1713 b provided in thechamber 1701 are connected to the source material supply portions 1711 aand 1711 b, respectively, through supply tubes and valves. The sourcematerial exhaust port 1714 is connected to the evacuation unit 1715through an exhaust tube, a valve, and a pressure controller.

A substrate holder 1716 with a heater is provided in the chamber, and asubstrate 1700 over which a film is deposited is provided over thesubstrate holder.

In the source material supply portions 1711 a and 1711 b, a precursor isformed from a solid source material or a liquid source material by usinga vaporizer, a heating unit, or the like. Alternatively, the sourcematerial supply portions 1711 a and 1711 b may supply a precursor in agas state.

Although two source material supply portions 1711 a and 1711 b areprovided in this example, the number of source material supply portionsis not limited thereto, and three or more source material supplyportions may be provided. The high-speed valves 1712 c and 1712 d can beaccurately controlled by time, and supply one of a precursor and aninert gas. The high-speed valves 1712 c and 1712 d are flow ratecontrollers for a precursor, and can also be referred to as flow ratecontrollers for an inert gas.

In the deposition apparatus illustrated in FIG. 5A, a thin film isformed over a surface of the substrate 1700 in the following manner: thesubstrate 1700 is transferred to be put on the substrate holder 1716;the chamber 1701 is sealed; the substrate 1700 is heated to a desiredtemperature (e.g., higher than or equal to 100° C. or higher than orequal to 150° C.) by heating the substrate holder 1716 with a heater;and supply of a precursor, evacuation with the evacuation unit 1715,supply of an inert gas, and evacuation with the evacuation unit 1715 arerepeated.

In the deposition apparatus illustrated in FIG. 5A, an insulating layerformed using an oxide (including a composite oxide) containing one ormore elements selected from hafnium, aluminum, tantalum, zirconium, andthe like can be deposited by selecting as appropriate a source material(e.g., a volatile organometallic compound) used for the source materialsupply portions 1711 a and 1711 b. Specifically, it is possible todeposit an insulating layer including hafnium oxide, an insulating layerincluding aluminum oxide, an insulating layer including hafniumsilicate, or an insulating layer including aluminum silicate.Alternatively, a thin film, e.g., a metal layer such as a tungsten layeror a titanium layer, or a nitride layer such as a titanium nitride layercan be deposited by selecting as appropriate a source material (e.g., avolatile organometallic compound) used for the source material supplyportions 1711 a and 1711 b.

For example, in the case where a hafnium oxide layer is formed with adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a precursor which is obtained byvaporizing liquid containing a solvent and a hafnium precursor compound(hafnium alkoxide or hafnium amide such astetrakis(dimethylamide)hafnium (TDMAH)), are used. In this case, thefirst precursor supplied from the source material supply portion 1711 ais TDMAH, and the second precursor supplied from the source materialsupply portion 1711 b is ozone. Note that the chemical formula oftetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of anothermaterial include tetrakis(ethylmethylamide)hafnium. Note that nitrogenhas a function of eliminating charge trap states. Therefore, when theprecursor contains nitrogen, a hafnium oxide film having low density ofcharge trap states can be deposited.

For example, in the case where an aluminum oxide layer is formed with adeposition apparatus utilizing an ALD method, two kinds of gases, i.e.,H₂O as an oxidizer and a precursor which is obtained by vaporizingliquid containing a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)), are used. In this case, the first precursorsupplied from the source material supply portion 1711 a is TMA, and thesecond precursor supplied from the source material supply portion 1711 bis H₂O. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃.Examples of another material liquid include tris(dimethylamide)aluminum,triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed with adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₃ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is deposited using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are sequentially introduced pluraltimes to form a tungsten film. Note that an SiH₄ gas may be used insteadof a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is deposited using a deposition apparatus employing ALD,an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural timesto form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are sequentiallyintroduced plural times to form a GaO layer, and then a Zn(CH₃)₂ gas andan O₃ gas are sequentially introduced plural times to form a ZnO layer.Note that the order of these layers is not limited to this example. Amixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or aGa—Zn—O layer may be formed by mixing these gases. Note that although anH₂O gas which is obtained by bubbling with an inert gas such as Ar maybe used instead of an O₃ gas, it is preferable to use an O₃ gas, whichdoes not contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may beused. Furthermore, a Zn(CH₃)₂ gas may be used.

<Multi-Chamber Manufacturing Apparatus>

FIG. 5B illustrates an example of a multi-chamber manufacturingapparatus including at least one deposition apparatus illustrated inFIG. 5A.

In the manufacturing apparatus illustrated in FIG. 5B, a stack of filmscan be successively deposited without exposure to the air, and entry ofimpurities is prevented and throughput is improved.

The manufacturing apparatus illustrated in FIG. 5B includes at least aload chamber 1702, a transfer chamber 1720, a pretreatment chamber 1703,a chamber 1701 which is a deposition chamber, and an unload chamber1706. Note that in order to prevent attachment of moisture, the chambersof the manufacturing apparatus (including the load chamber, thetreatment chamber, the transfer chamber, the deposition chamber, theunload chamber, and the like) are preferably filled with an inert gas(such as a nitrogen gas) whose dew point is controlled and furtherpreferably maintain reduced pressure.

Chambers 1704 and 1705 may be deposition apparatuses utilizing an ALDmethod like the chamber 1701, deposition apparatuses utilizing a plasmaCVD method, deposition apparatuses utilizing a sputtering method, ordeposition apparatuses utilizing a MOCVD method.

For example, an example in which a stack of films is formed under acondition that the chamber 1704 is a deposition apparatus utilizing aplasma CVD method and the chamber 1705 is a deposition apparatusutilizing an MOCVD method is described below.

Although FIG. 5B illustrates an example in which a top view of thetransfer chamber 1720 is a hexagon, a manufacturing apparatus in whichthe top surface shape is set to a polygon having more than six cornersand more chambers are connected depending on the number of layers of astack may be used. The top surface shape of the substrate is rectangularin FIG. 5B; however, there is no particular limitation on the topsurface shape of the substrate. Although FIG. 5B illustrates an exampleof the single wafer type, a batch-type deposition apparatus in whichfilms are deposited on a plurality of substrates at a time may be used.

<Formation of Insulating Layer 110>

First, the insulating layer 110 is deposited over the substrate 100. Theinsulating layer 110 can be formed by a plasma CVD method, a thermal CVDmethod (an MOCVD method, or an ALD method), a sputtering method, or thelike usingan oxide insulating film of aluminum oxide, magnesium oxide,silicon oxide, silicon oxynitride, silicon oxyfluoride, gallium oxide,germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, or the like, a nitrideinsulating film of silicon nitride, silicon nitride oxide, aluminumnitride, aluminum nitride oxide, or the like, or a mixed material of anyof these. Alternatively, a stack of the above materials may be used.

The use of a material that contains no hydrogen or a material thatcontains hydrogen at 1% or lower for the insulating layer 110 canprevent oxygen vacancies from being generated in the oxidesemiconductor, leading to stable operation of the transistor.

As the insulating layer 110, for example, a 100-nm-thick siliconoxynitride film can be formed by a plasma CVD method.

Next, first heat treatment may be performed to release water, hydrogen,or the like contained in the insulating layer 110. As a result, theconcentration of water, hydrogen, or the like contained in theinsulating layer 110 can be reduced. The heat treatment can reduce theamount of water, hydrogen, or the like diffused into a first oxideinsulating film that is to be formed later.

The temperature of the first heat treatment is higher than or equal to250° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 300° C. and lower than or equal to 650° C.,further preferably higher than or equal to 350° C. and lower than orequal to 550° C.

The first heat treatment is preferably performed under an inert gasatmosphere containing nitrogen or a rare gas such as helium, neon,argon, xenon, or krypton. Further, after heat treatment performed in aninert gas atmosphere, heat treatment may be additionally performed in anoxygen atmosphere or a dry air atmosphere (air whose dew point is lowerthan or equal to −80° C., preferably lower than or equal to −100° C. andfurther preferably lower than or equal to −120° C.). The treatment maybe performed under reduced pressure. Note that it is preferable thathydrogen, water, and the like not be contained in an inert gas andoxygen, like the dry air, and the dew point is preferably lower than orequal to −80° C. and further preferably lower than or equal to −100° C.The treatment time is preferably 30 seconds to 24 hours.

In the first heat treatment, instead of an electric furnace, any devicefor heating an object by heat conduction or heat radiation from aheating element, such as a resistance heating element, may be used. Forexample, an RTA (rapid thermal annealing) apparatus, such as a GRTA (gasrapid thermal annealing) apparatus or an LRTA (lamp rapid thermalannealing) apparatus, can be used. The LRTA apparatus is an apparatusfor heating an object to be processed by radiation of light (anelectromagnetic wave) emitted from a lamp, such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. The GRTA apparatus is anapparatus for the heat treatment using a high-temperature gas. As thehigh-temperature gas, an inert gas, such as nitrogen or a rare gas likeargon, is used.

<Formation of First Oxide Insulating Film and Oxide Semiconductor Film>

Then, the first oxide insulating film to be the oxide insulating layer121 later and the oxide semiconductor film to be the oxide semiconductorlayer 122 later are deposited over the insulating layer 110. The firstoxide insulating film and the oxide semiconductor film can be formed bya sputtering method, an MOCVD method, a PLD method, or the like, andespecially, a sputtering method is preferable. As a sputtering method,an RF sputtering method, a DC sputtering method, an AC sputteringmethod, or the like can be used. In addition, a facing-target-typesputtering method (also referred to as a counter-electrode-typesputtering method, a gas phase sputtering method, and a vapor depositionsputtering (VDSP) method) is used, whereby plasma damage at thedeposition can be reduced.

When the oxide semiconductor film is formed by a sputtering method, forexample, it is preferable that each chamber of the sputtering apparatusbe able to be evacuated to a high vacuum (approximately 5×10⁻⁷ Pa to1×10⁻⁴ Pa) by an adsorption vacuum pump such as a cryopump, and that thechamber be able to heat a substrate over which a film is to be depositedto 100° C. or higher, preferably 400° C. or higher, so that water andthe like acting as impurities in the oxide semiconductor layer 122 canbe removed as much as possible. Alternatively, a combination of a turbomolecular pump and a cold trap is preferably used to prevent back-flowof a gas containing a carbon component, moisture, or the like from anexhaust system into the chamber. Alternatively, a combination of a turbomolecular pump and a cryopump may be used as an exhaust system.

Not only high vacuum evacuation in a chamber but also high purity of asputtering gas is desirable to obtain a highly purified intrinsic oxidesemiconductor layer. As an oxygen gas or an argon gas used as asputtering gas, a highly purified gas having a dew point of −40° C. orlower, preferably −80° C. or lower and further preferably −100° C. orlower, is used, whereby moisture or the like can be prevented fromentering an oxide semiconductor film as much as possible.

As a sputtering gas, a rare gas (typically argon), oxygen, or a mixedgas of a rare gas and oxygen can be used as appropriate.

Note that for example, in the case where the oxide semiconductor film isdeposited by a sputtering method at a substrate temperature higher thanor equal to 20° C. and lower than or equal to 750° C., preferably higherthan or equal to 150° C. and lower than or equal to 450° C. and furtherpreferably higher than or equal to 200° C. and lower than or equal to420° C., the oxide semiconductor film can be a CAAC-OS film.

The material for the first oxide insulating film is desirably selectedso that the first oxide insulating film can have a lower electronaffinity than the oxide semiconductor film.

For example, when a sputtering method is used to deposit the first oxideinsulating film and the oxide semiconductor film, the first oxideinsulating film and the oxide semiconductor film can be successivelydeposited without being exposed to the air with the use of amulti-chamber sputtering apparatus. In that case, entry of unnecessaryimpurities and the like into the interface between the first oxideinsulating film and the oxide semiconductor film can be prevented andinterface states can be reduced accordingly. Thus, the electricalcharacteristics of a transistor can be stabilized, particularly in areliability test.

If the insulating layer 110 is damaged, the oxide semiconductor layer122, which is a main conduction path, can keep a distance from thedamaged part owing to the existence of the oxide insulating layer 121.Thus, the electrical characteristics of a transistor can be stabilized,particularly in a reliability test.

For example, as the first oxide insulating film, a 20-nm-thick oxideinsulating film which is deposited by a sputtering method using a targethaving an atomic ratio of In:Ga:Zn=1:3:4 can be used. In addition, asthe oxide semiconductor film, a 15-nm-thick oxide semiconductor filmwhich is deposited by a sputtering method using a target having anatomic ratio of In:Ga:Zn=1:1:1 can be used.

Note that the amount of oxygen vacancies in the oxide semiconductor filmcan be reduced by performing second heat treatment after the first oxideinsulating film and the oxide semiconductor film are deposited.

The temperature of the second heat treatment is preferably higher thanor equal to 250° C. and lower than the strain point of the substrate,further preferably higher than or equal to 300° C. and lower than orequal to 650° C., and still further preferably higher than or equal to350° C. and lower than or equal to 550° C.

The second heat treatment can be performed in a manner similar to thefirst heat treatment.

Note that the second heat treatment may be performed after etching forforming the oxide insulating layer 121 and the oxide semiconductor layer122 described later.

For example, after heat treatment is performed at 450° C. in a nitrogenatmosphere for one hour, heat treatment is performed at 450° C. in anoxygen atmosphere for one hour.

Through the above-described steps, oxygen vacancies and impurities suchas hydrogen and water in the oxide semiconductor film can be reduced.The oxide semiconductor film can have low density of localized states.

Note that high-density plasma irradiation using oxygen can exert aneffect equivalent to that exerted by heat treatment. Irradiation time islonger than or equal to 1 minute and shorter than or equal to 3 hours,preferably longer than or equal to 3 minutes and shorter than or equalto 2 hours, and further preferably longer than or equal to 5 minutes andshorter than or equal to 1 hour.

<Formation of First Conductive Film>

Next, a first conductive film used as a hard mask is formed over theoxide semiconductor film. The first conductive film can be formed by asputtering method, a chemical vapor deposition (CVD) method such as ametal organic chemical vapor deposition (MOCVD) method, a metal chemicalvapor deposition method, an atomic layer deposition (ALD) method, or aplasma-enhanced chemical vapor deposition (PECVD) method, an evaporationmethod, a pulsed laser deposition (PLD) method, or the like.

The first conductive film is preferably, for example, a conductive filmhaving a single-layer structure or a stacked-layer structure andcontaining a material selected from copper (Cu), tungsten (W),molybdenum (Mo), gold (Au), aluminum (Al), manganese (Mn), titanium(Ti), tantalum (Ta), nickel (Ni), chromium (Cr), lead (Pb), tin (Sn),iron (Fe), cobalt (Co), ruthenium (Ru), platinum (Pt), iridium (Ir), andstrontium (Sr), an alloy of such a material, or a compound containingsuch a material as its main component.

As the first conductive film, for example, a tungsten film having athickness of 20 nm to 100 nm can be formed by a sputtering method.

Although the first conductive film is formed as a hard mask in thisembodiment, one embodiment of the present invention is not limitedthereto, and an insulating film may be formed instead.

<Formation of Oxide Insulating Layer 121 and Oxide Semiconductor Layer122>

Next, a resist mask is formed over the first conductive film by alithography process. When the resist mask is formed, the firstconductive film is selectively etched using the resist mask to form ahard mask. After the resist over the hard mask is removed, the oxidesemiconductor film and the first oxide insulating film are selectivelyetched, whereby the island-shaped oxide semiconductor layer 122 andoxide insulating layer 121 are formed (see FIGS. 6A to 6C). Dry etchingcan be used here.

For example, the first oxide insulating film and the oxide semiconductorfilm are selectively etched using a resist mask and a hard mask with amethane gas and an argon gas as an etching gas, whereby the oxideinsulating layer 121 and the oxide semiconductor layer 122 can beformed. After the oxide insulating layer 121 and the oxide semiconductorlayer 122 are formed, the first conductive film is removed.

Note that the use of the first conductive film as a hard mask foretching of the oxide semiconductor layer can reduce edge roughness ofthe oxide semiconductor layer after the etching as compared with thecase of using a resist mask.

<Formation of Oxide Insulating Layer 123>

Next, an oxide insulating layer 123 a to be the oxide insulating layer123 is deposited over the oxide semiconductor layer 122 and theinsulating layer 110 (see FIGS. 7A to 7C). The oxide insulating layer123 a can be deposited in a manner similar to that of the oxidesemiconductor film and the first oxide insulating film. The materialscan be selected such that the electron affinity of the oxide insulatinglayer 123 a is smaller than that of the oxide semiconductor film. Notethat the oxide insulating layer 123 a may be processed using a resistmask or the gate electrode layer 160 as a mask.

For example, as the oxide insulating layer 123 a, a 5-nm-thick oxidesemiconductor film which is deposited by a sputtering method using atarget having an atomic ratio of In:Ga:Zn=1:3:2 can be used.

<Formation of Insulating Film 150 a>

Next, an insulating film 150 a to be the gate insulating layer 150 isformed over the oxide insulating layer 123 a. The insulating film 150 amay be a stack containing any of these materials. The insulating film150 a can be formed by a sputtering method, a CVD method (e.g., a plasmaCVD method, an MOCVD method, or an ALD method), an MBE method, or thelike. The insulating film 150 a can be formed by a method similar tothat of the insulating layer 110 as appropriate.

For example, as the insulating film 150 a, silicon oxynitride can bedeposited to a thickness of 10 nm by a plasma CVD method.

<Formation of Conductive Film 160 a>

Next, a conductive film 160 a to be the gate electrode layer 160 isdeposited over the insulating film 150 a (see FIGS. 8A to 8C). Theconductive film 160 a can be formed by a sputtering method, a CVD method(e.g., a plasma CVD method, an MOCVD method, or an ALD method), an MBEmethod, an evaporation method, a plating method, or the like. Theconductive film 160 a may be formed using a conductive film containingnitrogen or a stack including the above conductive film and a conductivefilm containing nitrogen.

For example, a stack of 10-nm-thick titanium nitride deposited by an ALDmethod and 150-nm-thick tungsten deposited by a metal CVD method can beused as the conductive film 160 a.

<Formation of Gate Electrode Layer 160 and Gate Insulating Layer 150>

Then, a resist mask is formed over the conductive film 160 a through alithography process. The conductive film 160 a is selectively etchedusing the resist mask, so that the gate electrode layer 160 is formed.After the resist over the gate electrode layer 160 is removed, theinsulating film 150 a and the oxide insulating layer 123 a areselectively etched using the gate electrode layer 160 as a mask, so thatthe gate insulating layer 150 and the oxide insulating layer 123 areformed (see FIGS. 9A to 9C).

<Modification Example of Gate Electrode Layer 160 and Gate InsulatingLayer 150>

Note that the formation method of the gate electrode layer 160 and thegate insulating layer 150 is not limited to the above method. Forexample, the gate electrode layer 160 and the gate insulating layer 150may be formed in such a manner that a groove portion is provided andthen an insulating film and a conductive film are embedded therein.

[Deposition of First Insulating Film]

First, a first insulating film to be an insulating layer 175 later isdeposited over the oxide insulating layer 123. The first insulating filmcan be deposited using a material and a method similar to those of theinsulating layer 110.

[Planarization of First Insulating Film]

Then, planarization treatment is performed on the first insulating film,so that an insulating layer 175 b is formed (see FIGS. 10A to 10C). Theplanarization treatment can be performed by a chemical mechanicalpolishing (CMP) method, a dry etching method, a reflow method, or thelike. In the case where the CMP method is used, a film whose compositionis different from that of the first insulating film is formed over thefirst insulating film, whereby the thickness of the insulating layer 175b in the substrate surface after the CMP treatment can be uniform.

[Formation of Groove Portion]

Next, a resist mask is formed over the planarized insulating layer 175 bby a lithography process. Note that the lithography process may beperformed after an organic film is applied to the insulating layer orafter an organic film is applied to the resist mask. The organic filmcan contain propylene glycolmonomethyl ether, ethyl lactate, or thelike. The use of the organic film leads to, in addition to ananti-reflection effect during light exposure, an improvement in adhesionbetween a resist and a film, an improvement in resolution, and the like.The organic film can be used in another process.

Note that in the case where a transistor having an extremely shortchannel length is formed, etching is performed using a resist mask thatis processed by a method suitable for micropatterning, such as electronbeam exposure, liquid immersion exposure, or extreme ultraviolet (EUV)exposure. Note that in the case of forming the resist mask by electronbeam exposure, a positive resist mask is used, so that an exposed regioncan be minimized and throughput can be improved. In the above manner, atransistor having a channel length of 100 nm or less, further, 30 nm orless, still further, 20 nm or less can be formed. Alternatively, minuteprocessing may be performed by an exposure technology which uses X-raysor the like.

With the use of the resist mask, groove processing is performed on theinsulating layer 175 b by a dry etching method until the oxideinsulating layer 123 is exposed. As a result of the processing, theinsulating layer 175 and the groove portion 174 are formed.

Note that the groove portion 174 is preferably perpendicular to asubstrate surface.

Note that the processing method of the groove portion 174 is not limitedto the above method. For example, not only the resist mask but also ahard mask may be used, or a half-tone mask may be used in a lithographyprocess to control the shape of the resist mask. Alternatively, theshape of the mask may be controlled by nanoimprint lithography or thelike. The nanoimprint lithography can be used in another process.

Next, the insulating film 150 a to be the gate insulating layer 150 isformed over the oxide insulating layer 123 and the insulating layer 175.

Next, the conductive film 160 a to be the gate electrode layer 160 isdeposited over the insulating film 150 a (see FIGS. 11A to 11C).

[Planarization Treatment]

Next, planarization treatment is performed (see FIGS. 12A to 12C). Theplanarization treatment can be performed by a CMP method, a dry etchingmethod, or the like. The planarization treatment may be terminated atthe time when the insulating film 150 a is exposed, or may be terminatedat the time when the insulating layer 175 is exposed. Accordingly, thegate electrode layer 160 and an insulating layer 150 b can be formed.

[Etch-Back Treatment of Insulating Layer 175]

Next, the insulating layer 175 and the insulating layer 150 b aresubjected to etch-back treatment by a dry etching method so that theoxide insulating layer 123 is exposed. Furthermore, part of the oxideinsulating layer 123 not overlapping with the gate electrode layer 160is etched, whereby the gate insulating layer 150 can be formed (seeFIGS. 13A to 13C). Note that the gate insulating layer 150 may remain ona side surface of the gate electrode layer.

With use of the above method, the gate electrode layer 160 and the gateinsulating layer 150 illustrated in FIGS. 13A to 13C can be formed.

<Formation of Sidewall Insulating Layer 176>

Next, a second insulating film to be the sidewall insulating layer 176is formed over the insulating layer 110, the oxide semiconductor layer122, and the gate electrode layer 160. Etching treatment by a dryetching method is performed on the second insulating film, whereby thesidewall insulating layer 176 which includes a region that is in contactwith side surfaces of the gate electrode layer 160 and the gateinsulating layer 150 is formed (see FIGS. 13A to 13C).

<Formation of Low-Resistance Region 127>

Next, a conductive layer 166 is deposited over the insulating layer 110,the oxide semiconductor layer 122, the gate electrode layer 160, and thesidewall insulating layer 176. As the conductive layer 166, a metalmaterial such as cobalt (Co), nickel (Ni), platinum (Pt), vanadium (V),chromium (Cr), and palladium (Pd) can be used. The metal elements arediffused into the oxide semiconductor layer 122 side and the oxideinsulating layer 123 side by performing heat treatment after deposition.

For example, cobalt can be deposited to a thickness of 10 nm as theconductive layer 166 by a sputtering method.

Next, a conductive layer 168 is deposited over the conductive layer 166(see FIGS. 14A to 14C). As the conductive layer 168, a metal materialsuch as titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V),niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten(W), rhodium (Rh), iridium (Ir), or cobalt (Co) is preferably used. Inthe case where heat treatment is performed after deposition of themetal, metal elements which are constituent elements of the oxidesemiconductor layer 122 and the oxide insulating layer 123 can bediffused into the conductive layer 168 side.

For example, titanium can be deposited to a thickness of 20 nm as theconductive layer 168 by a sputtering method.

Next, third heat treatment is preferably performed. The temperature ofthe heat treatment is higher than or equal to 250° C. and lower than thestrain point of the substrate, preferably higher than or equal to 400°C. and lower than or equal to 800° C., and further preferably higherthan or equal to 450° C. and lower than or equal to 650° C.

The third heat treatment can be performed in a manner similar to that ofthe first heat treatment. Note that an RTA method is preferably usedbecause the process time can be shortened.

The third heat treatment is preferably performed in an atmospherewithout oxygen.

For example, the third heat treatment can be performed at 500° C. in annitrogen atmosphere for 60 seconds by an RTA method.

Elements contained in the oxide semiconductor layer 122 and theconductive layer 166 are diffused by the third heat treatment (see FIGS.15A to 15C).

FIG. 55 is an enlarged view of part of FIG. 15B. A constituent element167 of the conductive layer 166 is diffused from the conductive layer166 to the oxide semiconductor layer 122 by the third heat treatment. Aconstituent element 169 in the oxide semiconductor layer 122 is diffusedfrom the oxide semiconductor layer 122 to the conductive layer 168 bythe third heat treatment. Such diffusions in the two directions canlower the energy barrier in diffusion as compared with the case whereany one of the conductive layer 166 and the conductive layer 168 existsalone. As a result, the diffusion of each element can be promoted andthe diffusion rate thereof can be increased. Note that the abovediffusion may involve the diffusion of a constituent element of theconductive layer 168.

As described above, when constituent elements of the oxide semiconductorlayer 122, the conductive layer 166, and the conductive layer 168 arediffused from one layer to another layer, the low-resistance region 127can be formed (see FIGS. 16A to 16C).

After the low-resistance region 127 is formed, the conductive layer 166and the conductive layer 168 are removed (see FIGS. 17A to 17C). Theconductive layers 166 and 168 may be removed by either a wet etchingmethod or a dry etching method. For example, the conductive layer 166and the conductive layer 168 can be removed by a mixture of ammoniumwater or a hydrogen peroxide solution and a mixture of hydrochloric acidand a hydrogen peroxide solution.

The low-resistance region 127 can include a region which is alloyed. Inthis case, the region which is alloyed can be left without being etched.

The region which is alloyed reaches the conductive layer 166 and theconductive layer 168 in some cases. For this reason, the distancebetween the top surface of the low-resistance region 127 and the topsurface of the substrate 100 and the distance between a top surface ofthe oxide semiconductor layer 122 and the top surface of the substrate100 are may be the same or different from each other.

The formed low-resistance region 127 can function as a source region anda drain region. The second region overlapping with the sidewallinsulating layer 176 includes part of the low-resistance region 127. Thethird region not overlapping with the gate electrode layer 160 and thesidewall insulating layer 176 is almost occupied by the low-resistanceregion 127. Therefore, since the proportion of the low-resistance region127 in the second region is lower than that in the third region, thesecond region has higher resistance than the third region and thus cansubstantially serve as an LDD region.

The above-described method for forming the low-resistance region 127 isextremely inexpensive and easy because an ion addition treatment or thelike is not performed. Furthermore, the resistance can be reduced ascompared with the case where the low-resistance region 127 is formed byan ion addition treatment. Therefore, the use of one embodiment of thepresent invention can improve the productivity of the semiconductordevice including a transistor.

Then, a third insulating film to be the insulating layer 180 is formed.The third insulating film can be formed in a manner similar to that ofthe insulating layer 110. It is desirable to perform planarization afterthe deposition of the third insulating film.

After that, the third insulating film is etched by a dry etching methodto form an opening.

Subsequently, a third conductive film to be the conductive layer 190 isformed in the opening, and then planarization treatment is performed,whereby the conductive layer 190 is formed.

Next, a fourth conductive film to be the conductive layer 195 is formedover the conductive layer 190. To form the conductive layer 195, thefourth conductive film is processed by a photolithography method, ananoimprinting method, or the like (see FIGS. 18A to 18C).

Accordingly, with one embodiment of the present invention, an extremelyminiaturized transistor whose channel length is less than or equal to100 nm, less than or equal to 30 nm, or less than or equal to 20 nm canbe manufactured stably.

Note that heat treatment may be performed at any time as required ineach step.

Although not mentioned in the above manufacturing method of thetransistor, a region with even lower resistance may be provided by ionaddition. For example, ions are added to the second region and the thirdregion of the oxide semiconductor layer 122 using the gate electrodelayer 160 as a mask. As a material to be added, hydrogen (H), fluorine(F), boron (B), phosphorus (P), helium (He), neon (Ne), argon (Ar),krypton (Kr), xenon (Xe), tungsten (W), aluminum (Al), molybdenum (Mo),indium (In), or the like can be used. The addition can be performed byan ion doping method, an ion implantation method, a plasma immersion ionimplantation method, high-density plasma treatment, or the like. Notethat an ion implantation method is preferable for miniaturizationbecause addition of impurities other than the predetermined ion can beinhibited. A large area can be effectively treated by an ion dopingmethod or a plasma immersion ion implantation method.

The ion accelerating voltage for the ion addition treatment ispreferably controlled in accordance with the ion species andimplantation depth. For example, the ion accelerating voltage can behigher than or equal to 0.5 kV and lower than or equal to 100 kV, orhigher than or equal to 3 kV and lower than or equal to 60 kV. The doseof the ions is preferably greater than or equal to 1×10¹² ions/cm² andless than or equal to 1×10¹⁷ ions/cm², further preferably greater thanor equal to 1×10¹³ ions/cm² and less than or equal to 5×10¹⁶ ions/cm².

By the ion addition treatment, oxygen vacancies are formed in the oxidesemiconductor layer 122 and the oxide insulating layer 123, so that thelow-resistance region 125 is formed in the second and third regions.Note that the low-resistance region 125 may be formed in the entiresecond region or in part of the second region.

By performing third heat treatment after the ion addition treatment,damage caused to a film during the ion addition treatment can berepaired. By the heat treatment, the material to which the ion is addedcan be diffused into the oxide insulating layer 121.

Modification Example 1 of Transistor 10: Transistor 11

A transistor 11 with a shape different from that of the transistor 10illustrated in FIGS. 1A to 1C will be described with reference to FIGS.19A to 19C.

FIGS. 19A to 19C are a top view and cross-sectional views of thetransistor 11. FIG. 19A is a top view of the transistor 11 and FIGS. 19Band 19C are cross-sectional views taken along dashed-dotted line B1-B2and dashed-dotted line B3-B4 in FIG. 19A, respectively.

The end portion of the low-resistance region 127 in the transistor 11 iswider than that in the transistor 10. As illustrated in FIGS. 19A to19C, the low-resistance region 127 may be included only in the oxidesemiconductor layer 122 or may be included also in the oxide insulatinglayer 123.

Like the transistor 10, the distance between a top surface of thelow-resistance region 127 and a top surface of the substrate 100 may bethe same as the distance between an upper end of the oxide semiconductorlayer 122 and the top surface of the substrate 100 (see FIG. 20B), ormay be larger or smaller than the distance between the upper end of theoxide semiconductor layer 122 and the top surface of the substrate 100.

With the structure of the transistor 11, a side wall portion in thechannel length direction can also be protected. Accordingly, thereliability of the transistor can be improved.

Modification Example 2 of Transistor 10: Transistor 12

A transistor 12 with a shape different from that of the transistor 10illustrated in FIGS. 1A to 1C will be described with reference to FIGS.21A to 21C.

FIGS. 21A to 21C are a top view and cross-sectional views of thetransistor 12. FIG. 21A is a top view of the transistor 12 and FIGS. 21Band 21C are cross-sectional views taken along dashed-dotted line C1-C2and dashed-dotted line C3-C4 in FIG. 21A, respectively.

The transistor 12 is different from the transistor 10 in that aconductive layer 165 is provided.

<Conductive Layer 165>

The conductive layer 165 can be formed using a material similar to thatof the gate electrode layer 160. Note that the conductive layer 165 maybe a single layer or a stacked layer.

The conductive layer 165 can have a function similar to that of the gateelectrode layer 160. The conductive layer 165 and the gate electrodelayer 160 may be configured to be applied with the same potential ordifferent potentials.

In the transistor 12 including the conductive layer 165, the insulatinglayer 110 can have a structure and a function similar to those of thegate insulating layer 150.

With the structure of the transistor 12, electrical characteristics(e.g., threshold voltage) of a transistor can be controlled.

Modification Example 3 of Transistor 10: Transistor 13

A transistor 13 with a shape different from that of the transistor 10illustrated in FIGS. 22A to 22C will be described with reference toFIGS. 22A to 22C.

FIGS. 22A to 22C are a top view and cross-sectional views of thetransistor 13. FIG. 22A is a top view of the transistor 13 and FIGS. 22Band 22C are cross-sectional views taken along dashed-dotted line D1-D2and dashed-dotted line D3-D4 in FIG. 22A, respectively.

The transistor 13 is different from the transistor 10 in that insulatinglayers 170 and 172 are provided.

<Insulating Layer 170>

The insulating layer 170 can contain oxygen (O), nitrogen (N), fluorine(F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga),germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium(Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. Forexample, the insulating layer 170 can contain one or more of aluminumoxide (AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)),silicon oxynitride (SiO_(x)N), silicon nitride oxide (SiN_(x)O_(y)),silicon nitride (SiN), gallium oxide (GaO_(x)), germanium oxide(GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanumoxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), andtantalum oxide (TaO_(x)).

An aluminum oxide (AlO_(x)) film is preferably included in theinsulating layer 170. The aluminum oxide film can prevent the passage ofboth oxygen and impurities such as hydrogen and moisture. Thus, duringand after the manufacturing process of the transistor, the aluminumoxide film can suitably function as a protective film that has effectsof preventing entry of impurities such as hydrogen and moisture, whichcause variations in the electrical characteristics of the transistor,into the oxide insulating layer 121, the oxide semiconductor layer 122,and the oxide insulating layer 123, preventing release of oxygen, whichis a main component, from the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123, andpreventing unnecessary release of oxygen from the insulating layer 110.

The insulating layer 170 is preferably a film having oxygen supplycapability. In the deposition of the insulating layer 170, a mixed layeris formed at an interface with a different oxide layer and oxygen issupplied to the mixed layer or the different oxide layer. The oxygen isdiffused into the oxide semiconductor layer by heat treatment performedafter that, and the oxygen can fill oxygen vacancies in the oxidesemiconductor layer; therefore, the transistor characteristics (e.g.,threshold voltage and reliability) can be improved.

Further, the insulating layer 170 may be a single layer or a stackedlayer. Alternatively, another insulating layer may be provided over orunder the insulating layer. For example, an insulating film containingone or more of magnesium oxide, silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide,yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide,hafnium oxide, and tantalum oxide can be used. The insulating layerpreferably contains oxygen more than that in the stoichiometriccomposition. Oxygen released from the insulating layer can be diffusedinto the channel formation region in the oxide semiconductor layer 122through the gate insulating layer 150 or the insulating layer 110, sothat oxygen vacancies formed in the channel formation region can befilled with the oxygen. In this manner, stable electricalcharacteristics of the transistor can be achieved.

<Insulating Layer 172>

The insulating layer 172 can contain oxygen (O), nitrogen (N), fluorine(F), aluminum (Al), magnesium (Mg), silicon (Si), gallium (Ga),germanium (Ge), yttrium (Y), zirconium (Zr), lanthanum (La), neodymium(Nd), hafnium (Hf), tantalum (Ta), titanium (Ti), or the like. Forexample, the insulating layer 172 can contain one or more of aluminumoxide (AlO_(x)), magnesium oxide (MgO_(x)), silicon oxide (SiO_(x)),silicon oxynitride (SiO_(x)N_(y)), silicon nitride oxide (SiN_(x)O_(y)),silicon nitride (SiN_(x)), gallium oxide (GaO_(x)), germanium oxide(GeO_(x)), yttrium oxide (YO_(x)), zirconium oxide (ZrO_(x)), lanthanumoxide (LaO_(x)), neodymium oxide (NdO_(x)), hafnium oxide (HfO_(x)), andtantalum oxide (TaO_(x)). The insulating layer 172 may be a stack of anyof the above materials.

An aluminum oxide film is preferably included in the insulating layer172. The aluminum oxide film can prevent the passage of both oxygen andimpurities such as hydrogen and moisture. Thus, during and after themanufacturing process of the transistor, the aluminum oxide film cansuitably function as a protective film that has effects of preventingentry of impurities such as hydrogen and moisture, which causevariations in the electrical characteristics of the transistor, into theoxide insulating layer 121, the oxide semiconductor layer 122, and theoxide insulating layer 123, preventing release of oxygen, which is amain component, from the oxide insulating layer 121, the oxidesemiconductor layer 122, and the oxide insulating layer 123, andpreventing release of oxygen from the insulating layer 110.

The insulating layer 172 can function as a protective film. Theinsulating layer 172 can protect the gate insulating layer 150 againstplasma damage. This can prevent an electron trap from being formed inthe vicinity of a channel.

<Manufacturing Method of Transistor 13>

Part of a manufacturing method of the transistor 13 will be describedwith reference to FIGS. 23A to 23C. For steps similar to those of themanufacturing method of the transistor 10, the description of themanufacturing method is referred to.

<Formation of Insulating Layer 172>

The insulating layer 172 is deposited over the oxide semiconductor layer122, the sidewall insulating layer 176, and the gate electrode layer160. The insulating layer 172 is preferably deposited by a metal organicchemical vapor deposition (MOCVD) method and an atomic layer deposition(ALD) method. Accordingly, damage to the oxide semiconductor layer 122and the gate insulating layer 150 can be inhibited and oxidation of thegate electrode layer can be inhibited.

The thickness of the insulating layer 172 is preferably greater than orequal to 1 nm and less than or equal to 30 nm, and further preferablygreater than or equal to 3 nm and less than or equal to 10 nm.

An ion may be added to the oxide semiconductor layer 122 after theinsulating layer 172 is deposited. Accordingly, the low-resistanceregion 127 can be formed while damage to the oxide semiconductor layer122 in the ion addition treatment can be reduced.

The insulating layer 172 may be used as deposited or may be processed bya lithography method, a nanoimprinting method, a dry etching method, orthe like.

<Formation of Insulating Layer 170>

Next, the insulating layer 170 is formed over the insulating layer 172.The insulating layer 170 may be a single layer or a stacked layer. Theinsulating layer 170 can be formed using a material, a method, and thelike similar to those of the insulating layer 110.

The insulating layer 170 is preferably an aluminum oxide film depositedby a sputtering method. A sputtering gas used for depositing thealuminum oxide film preferably contains an oxygen gas. The oxygen gas iscontained at 1 vol % or more and 100 vol % or less, preferably 4 vol %or more and 100 vol % or less and further preferably 10 vol % or moreand 100 vol % or less. When oxygen is contained at 1 vol % or more,excess oxygen can be supplied to the insulating layer or insulatinglayers in contact with the insulating layer. Furthermore, oxygen can beadded to the layers in contact with the layer.

For example, the insulating layer 170 having a thickness from 20 nm to40 nm can be deposited using aluminum oxide as a sputtering target and asputtering gas that contains an oxygen gas at 50 vol %.

Next, heat treatment is preferably performed. The temperature of theheat treatment is typically higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 250° C. and lower than or equal to 500° C. and further preferablyhigher than or equal to 300° C. and lower than or equal to 450° C. Bythe heat treatment, oxygen 173 added to an insulating layer (e.g., theinsulating layer 110) is diffused and moved to the oxide semiconductorlayer 122, and oxygen vacancies in the oxide semiconductor layer 122 canbe filled with the oxygen (see FIGS. 23A to 23C).

For example, the heat treatment can be performed at 400° C. in an oxygenatmosphere for one hour.

Note that heat treatment may be performed in other processes. Throughheat treatment, defects in a film can be repaired, and the density ofinterface states can be reduced.

<Oxygen Addition>

Oxygen is not necessarily added through the insulating layer 170. Oxygenmay be added to the insulating layer 110, the first oxide insulatingfilm, and the oxide insulating layer 123, or another insulating layer.As the oxygen that is added, at least one selected from oxygen radicals,oxygen atoms, oxygen atomic ions, oxygen molecular ions, and the like isused. As a method for adding the oxygen, an ion doping method, an ionimplantation method, a plasma immersion ion implantation method, or thelike can be used.

In the case of using an ion implantation method as the method for addingoxygen, oxygen atomic ions or oxygen molecular ions can be used. The useof oxygen molecular ions can reduce damage to a film to which oxygen isadded. Oxygen molecular ions are broken down into oxygen atomic ions atthe surface of the film to which oxygen is added, and the oxygen atomicions are added. Since energy for breaking oxygen molecules down intooxygen atoms is used, the energy per oxygen atomic ion in the case ofadding oxygen molecular ions to the film to which oxygen is added islower than that in the case of adding oxygen atomic ions to the film towhich oxygen is added. Therefore, damage to the film to which oxygen isadded can be reduced.

By using oxygen molecular ions, the energy of each oxygen atomic ioninjected to the film to which oxygen is added is lowered, which makesthe injected oxygen atomic ion be positioned in a shallow region.Accordingly, oxygen atoms easily move by later heat treatment, so thatmore oxygen can be supplied to the oxide semiconductor film.

In the case of injecting oxygen molecular ions, the energy per oxygenatomic ion is low as compared with the case of injecting oxygen atomicions. Thus, by using oxygen molecular ions for injection, theacceleration voltage can be increased and throughput can be increased.Moreover, by using oxygen molecular ions for injection, the dose can behalf of the amount that is necessary in the case of using oxygen atomicions. As a result, throughput can be increased.

In the case of adding oxygen to the film to which oxygen is added, it ispreferable that oxygen be added to the film to which oxygen is added sothat a peak of the concentration profile of oxygen atomic ions islocated in the film to which oxygen is added. In that case, theacceleration voltage for injection can be lowered as compared with thecase where oxygen atomic ions are injected, and damage to the film towhich oxygen is added can be reduced. In other words, defects in thefilm to which oxygen is added can be reduced, suppressing variations inelectrical characteristics of the transistor. Furthermore, in the casewhere oxygen is added to the film to which oxygen is added so that theamount of added oxygen atoms at the interface between the insulatinglayer 110 and the oxide insulating layer 121 is less than 1×10²¹atoms/cm³, less than 1×10²⁰ atoms/cm³, or less than 1×10¹⁹ atoms/cm³,the amount of oxygen added to the insulating layer 110 can be reduced.As a result, damage to the film to which oxygen is added can be reduced,suppressing variations in electrical characteristics of the transistor.

Plasma treatment (plasma immersion ion implantation method) in which thefilm to which oxygen is added is exposed to plasma generated in anatmosphere containing oxygen may be performed to add oxygen to the filmto which oxygen is added. As an example of the atmosphere containingoxygen, an atmosphere containing an oxidation gas such as oxygen, ozone,dinitrogen monoxide, or nitrogen dioxide can be given. Note that it ispreferable that the film to which oxygen is added be exposed to plasmagenerated in a state where bias is applied to the substrate 100 sidebecause the amount of oxygen added to the film to which oxygen is addedcan be increased. As an example of an apparatus with which such plasmatreatment is performed, an ashing apparatus or the like is given.

For example, oxygen molecular ions can be added to the insulating layer110 by an ion implantation method with a dose of 2×10¹⁶/cm² at anacceleration voltage of 60 kV.

The above process is applicable to the transistor 10 and othertransistors.

In this manner, the density of localized states of the oxidesemiconductor film is lowered, and thus a transistor with excellentelectrical characteristics can be manufactured. In addition, a highlyreliable transistor in which variations in electrical characteristicswith time or variations in electrical characteristics due to a stresstest are reduced can be manufactured.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 2 <Structure of Oxide Semiconductor>

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

An amorphous structure is generally thought to be isotropic and have nonon-uniform structure, to be metastable and not have fixed positions ofatoms, to have a flexible bond angle, and to have a short-range orderbut have no long-range order, for example.

In other words, a stable oxide semiconductor cannot be regarded as acompletely amorphous oxide semiconductor. Moreover, an oxidesemiconductor that is not isotropic (e.g., an oxide semiconductor thathas a periodic structure in a microscopic region) cannot be regarded asa completely amorphous oxide semiconductor. In contrast, an a-like OS,which is not isotropic, has an unstable structure that contains a void.Because of its instability, an a-like OS has physical properties similarto those of an amorphous oxide semiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalthat is classified into the space group R-3m is analyzed by anout-of-plane method, a peak appears at a diffraction angle (2θ) ofaround 31θ as shown in FIG. 24A. This peak is derived from the (009)plane of the InGaZnO₄ crystal, which indicates that crystals in theCAAC-OS have c-axis alignment, and that the c-axes are aligned in adirection substantially perpendicular to a surface over which theCAAC-OS film is formed (also referred to as a formation surface) or thetop surface of the CAAC-OS film. Note that a peak sometimes appears at a2θ of around 36° in addition to the peak at a 2θ of around 31°. The peakat a 2θ of around 36° is derived from a crystal structure that isclassified into the space group Fd-3m; thus, this peak is preferably notexhibited in a CAAC-OS.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on the CAAC-OS in a directionparallel to the formation surface, a peak appears at a 2θ of around 56°.This peak is attributed to the (110) plane of the InGaZnO₄ crystal. Whenanalysis (ϕ scan) is performed with 2θ fixed at around 56° and with thesample rotated using a normal vector to the sample surface as an axis (ϕaxis), as shown in FIG. 24B, a peak is not clearly observed. Incontrast, in the case where single crystal InGaZnO₄ is subjected to ϕscan with 2θ fixed at around 56°, as shown in FIG. 24C, six peaks whichare derived from crystal planes equivalent to the (110) plane areobserved. Accordingly, the structural analysis using XRD shows that thedirections of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the formation surface of the CAAC-OS, a diffraction pattern(also referred to as a selected-area electron diffraction pattern) shownin FIG. 24D can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 24E shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 24E, a ring-like diffraction pattern isobserved. Thus, the electron diffraction using an electron beam with aprobe diameter of 300 nm also indicates that the a-axes and b-axes ofthe pellets included in the CAAC-OS do not have regular orientation. Thefirst ring in FIG. 24E is considered to be derived from the (010) plane,the (100) plane, and the like of the InGaZnO₄ crystal. The second ringin FIG. 24E is considered to be derived from the (110) plane and thelike.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, even in thehigh-resolution TEM image, a boundary between pellets, that is, a grainboundary is not clearly observed in some cases. Thus, in the CAAC-OS, areduction in electron mobility due to the grain boundary is less likelyto occur.

FIG. 25A shows a high-resolution TEM image of a cross section of theCAAC-OS which is observed from a direction substantially parallel to thesample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. TheCs-corrected high-resolution TEM image can be observed with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 25A shows pellets in which metal atoms are arranged in a layeredmanner. FIG. 25A proves that the size of a pellet is greater than orequal to 1 nm or greater than or equal to 3 nm. Therefore, the pelletcan also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OScan also be referred to as an oxide semiconductor including c-axisaligned nanocrystals (CANC). A pellet reflects unevenness of a formationsurface or a top surface of the CAAC-OS, and is parallel to theformation surface or the top surface of the CAAC-OS.

FIGS. 25B and 25C show Cs-corrected high-resolution TEM images of aplane of the CAAC-OS observed from a direction substantiallyperpendicular to the sample surface. FIGS. 25D and 25E are imagesobtained through image processing of FIGS. 25B and 25C. The method ofimage processing is as follows. The image in FIG. 25B is subjected tofast Fourier transform (FFT), so that an FFT image is obtained. Then,mask processing is performed such that a range of from 2.8 nm⁻¹ to 5.0nm⁻¹ from the origin in the obtained FFT image remains. After the maskprocessing, the FFT image is processed by inverse fast Fourier transform(IFFT) to obtain a processed image. The image obtained in this manner iscalled an FFT filtering image. The FFT filtering image is a Cs-correctedhigh-resolution TEM image from which a periodic component is extracted,and shows a lattice arrangement.

In FIG. 25D, a portion where a lattice arrangement is broken is denotedwith a dashed line. A region surrounded by a dashed line is one pellet.The portion denoted with the dashed line is a junction of pellets. Thedashed line draws a hexagon, which means that the pellet has a hexagonalshape. Note that the shape of the pellet is not always a regular hexagonbut is a non-regular hexagon in many cases.

In FIG. 25E, a dotted line denotes a portion where the direction of alattice arrangement changes between a region with a regular latticearrangement and another region with a regular lattice arrangement, and adashed line denotes the change in the direction of the latticearrangement. A clear crystal grain boundary cannot be observed even inthe vicinity of the dotted line. When a lattice point in the vicinity ofthe dotted line is regarded as a center and surrounding lattice pointsare joined, a distorted hexagon, pentagon, and/or heptagon can beformed, for example. That is, a lattice arrangement is distorted so thatformation of a crystal grain boundary is inhibited. This is probablybecause the CAAC-OS can tolerate distortion owing to a low density ofthe atomic arrangement in an a-b plane direction, an interatomic bonddistance changed by substitution of a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets(nanocrystals) are connected in an a-b plane direction, and the crystalstructure has distortion. For this reason, the CAAC-OS can also bereferred to as an oxide semiconductor including a c-axis-aligneda-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry ofimpurities, formation of defects, or the like might decrease thecrystallinity of an oxide semiconductor. This means that the CAAC-OS hassmall amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. For example, an oxygen vacancyin the oxide semiconductor might serve as a carrier trap or serve as acarrier generation source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancies isan oxide semiconductor with low carrier density (specifically, lowerthan 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, and furtherpreferably lower than 1×10¹⁰/cm³, and is higher than or equal to1×10⁻⁹/cm³). Such an oxide semiconductor is referred to as a highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor. A CAAC-OS has a low impurity concentration and a lowdensity of defect states. Thus, the CAAC-OS can be referred to as anoxide semiconductor having stable characteristics.

<nc-OS>

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. When the structure of an nc-OSis analyzed by an out-of-plane method, a peak indicating orientationdoes not appear. That is, a crystal of an nc-OS does not haveorientation.

For example, when an electron beam with a probe diameter of 50 nm isincident on a 34-nm-thick region of thinned nc-OS including an InGaZnO₄crystal in a direction parallel to the formation surface, a ring-shapeddiffraction pattern (a nanobeam electron diffraction pattern) shown inFIG. 26A is observed. FIG. 26B shows a diffraction pattern obtained whenan electron beam with a probe diameter of 1 nm is incident on the samesample. As shown in FIG. 26B, a plurality of spots are observed in aring-like region. In other words, ordering in an nc-OS is not observedwith an electron beam with a probe diameter of 50 nm but is observedwith an electron beam with a probe diameter of 1 nm.

Furthermore, an electron diffraction pattern in which spots are arrangedin an approximately hexagonal shape is observed in some cases as shownin FIG. 26C when an electron beam having a probe diameter of 1 nm isincident on a region with a thickness of less than 10 nm. This meansthat an nc-OS has a well-ordered region, i.e., a crystal, in the rangeof less than 10 nm in thickness. Note that an electron diffractionpattern having regularity is not observed in some regions becausecrystals are aligned in various directions.

FIG. 26D shows a Cs-corrected high-resolution TEM image of a crosssection of an nc-OS observed from the direction substantially parallelto the formation surface. In a high-resolution TEM image, an nc-OS has aregion in which a crystal part is observed, such as the part indicatedby additional lines in FIG. 26D, and a region in which a crystal part isnot clearly observed. In most cases, the size of a crystal part includedin the nc-OS is greater than or equal to 1 nm and less than or equal to10 nm, in particular, greater than or equal to 1 nm and less than orequal to 3 nm. Note that an oxide semiconductor including a crystal partwhose size is greater than 10 nm and less than or equal to 100 nm issometimes referred to as a microcrystalline oxide semiconductor. In ahigh-resolution TEM image of the nc-OS, for example, a grain boundary isnot clearly observed in some cases. Note that there is a possibilitythat the origin of the nanocrystal is the same as that of a pellet in aCAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as apellet in the following description.

As described above, in the nc-OS, a microscopic region (e.g., a regionwith a size greater than or equal to 1 nm and less than or equal to 10nm, in particular, a region with a size greater than or equal to 1 nmand less than or equal to 3 nm) has a periodic atomic arrangement. Thereis no regularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not ordered.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedto an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-Like OS>

An a-like OS has a structure intermediate between those of the nc-OS andthe amorphous oxide semiconductor.

FIGS. 27A and 27B are high-resolution cross-sectional TEM images of ana-like OS. FIG. 27A is the high-resolution cross-sectional TEM image ofthe a-like OS at the start of the electron irradiation. FIG. 27B is thehigh-resolution cross-sectional TEM image of a-like OS after theelectron (e⁻) irradiation at 4.3×10⁸ e⁻/nm². FIGS. 27A and 27B show thatstripe-like bright regions extending vertically are observed in thea-like OS from the start of the electron irradiation. It can be alsofound that the shape of the bright region changes after the electronirradiation. Note that the bright region is presumably a void or alow-density region.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared to aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each ofthe samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure inwhich nine layers including three In—O layers and six Ga—Zn—O layers arestacked in the c-axis direction. The distance between the adjacentlayers is equivalent to the lattice spacing on the (009) plane (alsoreferred to as d value). The value is calculated to be 0.29 nm fromcrystal structural analysis. Accordingly, a portion where the spacingbetween lattice fringes is greater than or equal to 0.28 nm and lessthan or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ inthe following description. Each of lattice fringes corresponds to thea-b plane of the InGaZnO₄ crystal.

FIG. 28 shows change in the average size of crystal parts (at 22 pointsto 30 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 28 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose in obtaining TEM images, for example. As shownin FIG. 28, a crystal part of approximately 1.2 nm (also referred to asan initial nucleus) at the start of TEM observation grows to a size ofapproximately 1.9 nm at a cumulative electron (e⁻) dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 28, thecrystal part sizes in an nc-OS and a CAAC-OS are approximately 1.3 nmand approximately 1.8 nm, respectively, regardless of the cumulativeelectron dose. For the electron beam irradiation and TEM observation, aHitachi H-9000NAR transmission electron microscope was used. Theconditions of electron beam irradiation were as follows: theaccelerating voltage was 300 kV; the current density was 6.7×10⁵e⁻/(nm²·s); and the diameter of irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS is sometimesinduced by electron irradiation. In contrast, in the nc-OS and theCAAC-OS, growth of the crystal part is hardly induced by electronirradiation. Therefore, the a-like OS has an unstable structure ascompared to the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that in the case where an oxide semiconductor having a certaincomposition does not exist in a single crystal structure, single crystaloxide semiconductors with different compositions are combined at anadequate ratio, which makes it possible to calculate density equivalentto that of a single crystal oxide semiconductor with the desiredcomposition. The density of a single crystal oxide semiconductor havingthe desired composition can be calculated using a weighted averageaccording to the combination ratio of the single crystal oxidesemiconductors with different compositions. Note that it is preferableto use as few kinds of single crystal oxide semiconductors as possibleto calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer film including two or more films of an amorphous oxidesemiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.

Embodiment 3

In this embodiment, an example of a circuit including the transistor ofone embodiment of the present invention will be described with referenceto drawings.

<Cross-Sectional Structure>

FIG. 29A is a cross-sectional view of a semiconductor device of oneembodiment of the present invention. In FIG. 29A, X1-X2 direction andY1-Y2 direction represent a channel length direction and a channel widthdirection, respectively. The semiconductor device illustrated in FIG.29A includes a transistor 2200 using a first semiconductor material in alower portion and a transistor 2100 using a second semiconductormaterial in an upper portion. In FIG. 29A, an example is described inwhich the transistor described in the above embodiment as an example isused as the transistor 2100 using the second semiconductor material. Across-sectional view of the transistors in a channel length direction ison the left side of a dashed-dotted line, and a cross-sectional view ofthe transistors in a channel width direction is on the right side of thedashed-dotted line.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material can be a semiconductormaterial other than an oxide semiconductor (examples of such asemiconductor material include silicon (including strained silicon),germanium, silicon germanium, silicon carbide, gallium arsenide,aluminum gallium arsenide, indium phosphide, gallium nitride, and anorganic semiconductor), and the second semiconductor material can be anoxide semiconductor. A transistor using a material other than an oxidesemiconductor, such as single crystal silicon, can operate at high speedeasily. In contrast, a transistor using an oxide semiconductor anddescribed in the above embodiment as an example can have a smallsubthreshold value (S value) and a minute structure. Furthermore, thetransistor can operate at a high speed because of its high switchingspeed and has low leakage current because of its low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channeltransistor, and an appropriate transistor may be used in accordance witha circuit. Furthermore, the specific structure of the semiconductordevice, such as the material or the structure used for the semiconductordevice, is not necessarily limited to those described here except forthe use of the transistor of one embodiment of the present inventionwhich uses an oxide semiconductor.

FIG. 29A illustrates a structure in which the transistor 2100 isprovided over the transistor 2200 with insulators 2201 and 2207 providedtherebetween. A plurality of wirings 2202 are provided between thetransistor 2200 and the transistor 2100. Furthermore, wirings andelectrodes provided over and under the insulators are electricallyconnected to each other through a plurality of plugs 2203 embedded inthe insulators. An insulator 2204 covering the transistor 2100 and awiring 2205 over the insulator 2204 are provided.

The stack of the two kinds of transistors reduces the area occupied bythe circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is usedfor the transistor 2200 provided in a lower portion, hydrogen in aninsulator provided in the vicinity of the semiconductor film of thetransistor 2200 terminates dangling bonds of silicon; accordingly, thereliability of the transistor 2200 can be improved. Meanwhile, in thecase where an oxide semiconductor is used for the transistor 2100provided in an upper portion, hydrogen in an insulator provided in thevicinity of the semiconductor film of the transistor 2100 becomes afactor of generating carriers in the oxide semiconductor; thus, thereliability of the transistor 2100 might be decreased. Therefore, in thecase where the transistor 2100 using an oxide semiconductor is providedover the transistor 2200 using a silicon-based semiconductor material,providing the insulator 2207 having a function of preventing diffusionof hydrogen between the transistors 2100 and 2200 is particularlyeffective. The insulator 2207 makes hydrogen remain in the lowerportion, thereby improving the reliability of the transistor 2200. Inaddition, since the insulator 2207 suppresses diffusion of hydrogen fromthe lower portion to the upper portion, the reliability of thetransistor 2100 can also be improved.

The insulator 2207 can be, for example, formed using aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, oryttria-stabilized zirconia (YSZ).

Furthermore, a blocking film having a function of preventing diffusionof hydrogen is preferably formed over the transistor 2100 to cover thetransistor 2100 including an oxide semiconductor film. For the blockingfilm, a material that is similar to that of the insulator 2207 can beused, and in particular, an aluminum oxide film is preferably used. Thealuminum oxide film has a high shielding (blocking) effect of preventingpenetration of both oxygen and impurities such as hydrogen and moisture.Thus, by using the aluminum oxide film as the blocking film covering thetransistor 2100, release of oxygen from the oxide semiconductor filmincluded in the transistor 2100 can be prevented and entry of water andhydrogen into the oxide semiconductor film can be prevented. Note thatas the block film, the insulator 2204 having a stacked-layer structuremay be used, or the block film may be provided under the insulator 2204.

Note that the transistor 2200 can be a transistor of various typeswithout being limited to a planar type transistor. For example, thetransistor 2200 can be a fin-type transistor, a tri-gate transistor, orthe like. An example of a cross-sectional view in this case isillustrated in FIG. 29D. An insulator 2212 is provided over asemiconductor substrate 2211. The semiconductor substrate 2211 includesa projecting portion with a thin tip (also referred to a fin). Note thatan insulator may be provided over the projecting portion. The projectingportion does not necessarily have the thin tip; a projecting portionwith a cuboid-like projecting portion and a projecting portion with athick tip are permitted, for example. A gate insulator 2214 is providedover the projecting portion of the semiconductor substrate 2211, and agate electrode 2213 is provided over the gate insulator 2214. Source anddrain regions 2215 are formed in the semiconductor substrate 2211. Notethat here is shown an example in which the semiconductor substrate 2211includes the projecting portion; however, a semiconductor device of oneembodiment of the present invention is not limited thereto. For example,a semiconductor region having a projecting portion may be formed byprocessing an SOI substrate.

<Circuit Configuration Example>

In the above structure, electrodes of the transistors 2100 and 2200 canbe connected as appropriate; thus, a variety of circuits can be formed.Examples of circuit configurations which can be achieved by using asemiconductor device of one embodiment of the present invention aredescribed below.

<CMOS Inverter Circuit>

A circuit diagram in FIG. 29B shows a configuration of a CMOS inverterin which the p-channel transistor 2200 and the n-channel transistor 2100are connected to each other in series and in which gates of them areconnected to each other.

<CMOS Analog Switch>

A circuit diagram in FIG. 29C shows a configuration in which sources ofthe transistors 2100 and 2200 are connected to each other and drains ofthe transistors 2100 and 2200 are connected to each other. In FIG. 29A,X1-X2 direction and Y1-Y2 direction represent a channel length directionand a channel width direction, respectively. With such a configuration,the transistors can function as a CMOS analog switch.

<Memory Device Example>

An example of a semiconductor device (memory device) which includes thetransistor of one embodiment of the present invention, which can retainstored data even when not powered, and which has an unlimited number ofwrite cycles is shown in FIGS. 30A to 30C.

The semiconductor device illustrated in FIG. 30A includes a transistor3200 using a first semiconductor material, a transistor 3300 using asecond semiconductor material, and a capacitor 3400. As the transistor3300, the transistor described in Embodiment 1 can be used.

FIG. 30B is a cross-sectional view of the semiconductor device shown inFIG. 30A. The semiconductor device in the cross-sectional view has astructure in which the transistor 3300 is provided with a back gate;however, a structure without a back gate may be employed.

The transistor 3300 is a transistor in which a channel is formed in asemiconductor including an oxide semiconductor. Since the off-statecurrent of the transistor 3300 is low, stored data can be retained for along period. In other words, power consumption can be sufficientlyreduced because a semiconductor memory device in which refresh operationis unnecessary or the frequency of refresh operation is extremely lowcan be provided.

In FIG. 30A, a first wiring 3001 is electrically connected to a sourceelectrode of the transistor 3200. A second wiring 3002 is electricallyconnected to a drain electrode of the transistor 3200. A third wiring3003 is electrically connected to one of a source electrode and a drainelectrode of the transistor 3300. A fourth wiring 3004 is electricallyconnected to a gate electrode of the transistor 3300. A gate electrodeof the transistor 3200 is electrically connected to the other of thesource electrode and the drain electrode of the transistor 3300 and oneelectrode of the capacitor 3400. The fifth wiring 3005 is electricallyconnected to the other electrode of the capacitor 3400. Although thecapacitor 3400 is provided over the transistor 3300 in FIG. 30B, thecapacitor 3400 may be provided between the transistor 3200 and thetransistor 3300.

The semiconductor device in FIG. 30A has a feature that the potential ofthe gate electrode of the transistor 3200 can be retained, and thusenables writing, retaining, and reading of data as follows.

Writing and retaining of data are described. First, the potential of thefourth wiring 3004 is set to a potential at which the transistor 3300 isturned on, so that the transistor 3300 is turned on. Accordingly, thepotential of the third wiring 3003 is supplied to the gate of thetransistor 3200 and the capacitor 3400. That is, a predetermined chargeis supplied to the gate of the transistor 3200 (writing). Here, one oftwo kinds of charges providing different potential levels (hereinafterreferred to as a low-level charge and a high-level charge) is supplied.After that, the potential of the fourth wiring 3004 is set to apotential at which the transistor 3300 is turned off, so that thetransistor 3300 is turned off. Thus, the charge supplied to the gate ofthe transistor 3200 is held (retaining).

Since the off-state current of the transistor 3300 is extremely low, thecharge of the gate of the transistor 3200 is retained for a long time.

Next, reading of data is described. An appropriate potential (a readingpotential) is supplied to the fifth wiring 3005 while a predeterminedpotential (a constant potential) is supplied to the first wiring 3001,whereby the potential of the second wiring 3002 varies depending on theamount of charge retained in the gate electrode of the transistor 3200.This is because in the case of using an n-channel transistor as thetransistor 3200, an apparent threshold voltage V_(th) _(_) _(H) at thetime when the high-level charge is given to the gate electrode of thetransistor 3200 is lower than an apparent threshold voltage V_(th) _(_)_(L) at the time when the low-level charge is given to the gateelectrode of the transistor 3200. Here, an apparent threshold voltagerefers to the potential of the fifth wiring 3005 which is needed to turnon the transistor 3200. Thus, the potential of the fifth wiring 3005 isset to a potential V₀ which is between V_(th) _(_) _(H) and V_(th) _(_)_(L), whereby charge supplied to the gate electrode of the transistor3200 can be determined. For example, in the case where the high-levelcharge is supplied to the gate of the transistor 3200 in writing and thepotential of the fifth wiring 3005 is V₀ (>V_(th) _(_) _(H)), thetransistor 3200 is turned on. In the case where the low-level charge issupplied to the gate of the transistor 3200 in writing, even when thepotential of the fifth wiring 3005 is V₀ (<V_(th) _(_) _(L)), thetransistor 3200 remains off. Thus, the data retained in the gate of thetransistor 3200 can be read by determining the potential of the secondwiring 3002.

Note that in the case where memory cells are arrayed to be used, it isnecessary that only data of a desired memory cell be able to be read.For example, the fifth wiring 3005 of memory cells from which data isnot read may be supplied with a potential at which the transistor 3200is turned off regardless of the potential of the gate, that is, apotential lower than V_(th) _(_) _(H), whereby only data of a desiredmemory cell can be read. Alternatively, the fifth wiring 3005 of thememory cells from which data is not read may be supplied with apotential at which the transistor 3200 is turned on regardless of thepotential of the gate, that is, a potential higher than V_(th) _(_)_(L), whereby only data of a desired memory cell can be read.

The semiconductor device shown in FIG. 30C is different from thesemiconductor device shown in FIG. 30A in that the transistor 3200 isnot provided. Also in this case, writing and holding of data can beperformed in a manner similar to the above.

Next, reading of data is described. When the transistor 3300 is turnedon, the third wiring 3003 which is in a floating state and the capacitor3400 are electrically connected to each other, and the charge isredistributed between the third wiring 3003 and the capacitor 3400. As aresult, the potential of the third wiring 3003 is changed. The amount ofchange in the potential of the third wiring 3003 varies depending on thepotential of the first terminal of the capacitor 3400 (or the chargeaccumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the chargeredistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potentialof the first terminal of the capacitor 3400, C is the capacitance of thecapacitor 3400, C_(B) is the capacitance component of the third wiring3003, and V_(B0) is the potential of the third wiring 3003 before thecharge redistribution. Thus, it can be found that, assuming that thememory cell is in either of two states in which the potential of thefirst terminal of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potentialof the third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of thethird wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+c×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with apredetermined potential, data can be read.

In this case, a transistor including the first semiconductor materialmay be used for a driver circuit for driving a memory cell, and atransistor including the second semiconductor material may be stackedover the driver circuit as the transistor 3300.

When including a transistor in which a channel formation region isformed using an oxide semiconductor and which has an extremely lowoff-state current, the semiconductor device described in this embodimentcan retain stored data for an extremely long time. In other words,refresh operation becomes unnecessary or the frequency of the refreshoperation can be extremely low, which leads to a sufficient reduction inpower consumption. Moreover, stored data can be retained for a long timeeven when power is not supplied (note that a potential is preferablyfixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. Unlike in a conventional nonvolatile memory,for example, it is not necessary to inject and extract electrons intoand from a floating gate; thus, a problem such as deterioration of agate insulating layer is not caused. That is, the semiconductor deviceof the disclosed invention does not have a limit on the number of timesdata can be rewritten, which is a problem of a conventional nonvolatilememory, and the reliability thereof is drastically improved.Furthermore, data is written depending on the state of the transistor(on or off), whereby high-speed operation can be easily achieved.

With the use of the semiconductor device described in this embodiment, amemory device with low power consumption and high capacity (e.g., 1terabit or more) can be manufactured.

Note that in this specification, it might be possible for those skilledin the art to constitute one embodiment of the invention even whenportions to which all the terminals of an active element (e.g., atransistor or a diode), a passive element (e.g., a capacitor or aresistor), or the like are connected are not specified. In other words,one embodiment of the invention can be clear even when connectionportions are not specified. Further, in the case where a connectionportion is disclosed in this specification and the like, it can bedetermined that one embodiment of the invention in which a connectionportion is not specified is disclosed in this specification and thelike, in some cases. In particular, in the case where the number ofportions to which the terminal is connected might be plural, it is notnecessary to specify the portions to which the terminal is connected.Therefore, it might be possible to constitute one embodiment of theinvention by specifying only portions to which some of terminals of anactive element (e.g., a transistor or a diode), a passive element (e.g.,a capacitor or a resistor), or the like are connected.

Note that in this specification, it might be possible for those skilledin the art to specify the invention when at least the connection portionof a circuit is specified. Alternatively, it might be possible for thoseskilled in the art to specify the invention when at least a function ofa circuit is specified. In other words, when a function of a circuit isspecified, one embodiment of the invention can be clear. Furthermore, itcan be determined that one embodiment of the invention whose function isspecified is disclosed in this specification and the like. Therefore,when a connection portion of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a function is notspecified, and one embodiment of the invention can be constituted.Alternatively, when a function of a circuit is specified, the circuit isdisclosed as one embodiment of the invention even when a connectionportion is not specified, and one embodiment of the invention can beconstituted.

Note that in this specification, in a diagram or a text described in oneembodiment, it is possible to take out part of the diagram or the textand constitute an embodiment of the invention. Thus, in the case where adiagram or a text related to a certain portion is described, the contexttaken out from part of the diagram or the text is also disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, for example, in a diagram or text in which oneor more active elements (e.g., transistors or diodes), wirings, passiveelements (e.g., capacitors or resistors), conductive layers, insulatinglayers, semiconductors, organic materials, inorganic materials,components, devices, operating methods, manufacturing methods, or thelike are described, part of the diagram or the text is taken out, andone embodiment of the invention can be constituted. For example, from acircuit diagram in which N circuit elements (e.g., transistors orcapacitors; N is an integer) are provided, it is possible to constituteone embodiment of the invention by taking out M circuit elements (e.g.,transistors or capacitors; M is an integer, where M<N). As anotherexample, it is possible to constitute one embodiment of the invention bytaking out M layers (M is an integer, where M<N) from a cross-sectionalview in which N layers (N is an integer) are provided. As anotherexample, it is possible to constitute one embodiment of the invention bytaking out M elements (M is an integer, where M<N) from a flow chart inwhich N elements (N is an integer) are provided.

<Imaging Device>

An imaging device of one embodiment of the present invention isdescribed below.

FIG. 31A is a plan view illustrating an example of an imaging device 200of one embodiment of the present invention. The imaging device 200includes a pixel portion 210 and peripheral circuits for driving thepixel portion 210 (peripheral circuits 260, 270, 280, and 290). Thepixel portion 210 includes a plurality of pixels 211 arranged in amatrix with p rows and q columns (p and q are each an integer greaterthan or equal to 2). The peripheral circuits 260, 270, 280, and 290 areeach connected to a plurality of pixels 211 and each have a function ofsupplying a signal for driving the plurality of pixels 211. In thisspecification, in some cases, a “peripheral circuit” or a “drivercircuit” indicates all of the peripheral circuits 260, 270, 280, and290. For example, the peripheral circuit 260 can be regarded as part ofthe peripheral circuit.

The peripheral circuit includes at least one of a logic circuit, aswitch, a buffer, an amplifier circuit, and a converter circuit. Theperipheral circuit may be provided over a substrate where the pixelportion 210 is formed. A semiconductor device such as an IC chip may beused as part or the whole of the peripheral circuit. Note that as theperipheral circuit, one or more of the peripheral circuits 260, 270,280, and 290 may be omitted.

As illustrated in FIG. 31B, the pixels 211 may be provided to beinclined in the pixel portion 210 included in the imaging device 200.When the pixels 211 are obliquely arranged, the distance between pixels(pitch) can be shortened in the row direction and the column direction.Accordingly, the quality of an image taken with the imaging device 200can be improved.

Configuration Example 1 of Pixel

The pixel 211 included in the imaging device 200 is formed with aplurality of subpixels 212, and each subpixel 212 is combined with afilter which transmits light in a specific wavelength range (colorfilter), whereby data for achieving color image display can be obtained.

FIG. 32A is a plan view showing an example of the pixel 211 with which acolor image is obtained. The pixel 211 illustrated in FIG. 32A includesa subpixel 212 provided with a color filter transmitting light in a red(R) wavelength range (also referred to as a subpixel 212R), a subpixel212 provided with a color filter transmitting light in a green (G)wavelength range (also referred to as a subpixel 212G), and a subpixel212 provided with a color filter transmitting light in a blue (B)wavelength range (also referred to as a subpixel 212B). The subpixel 212can function as a photosensor.

The subpixel 212 (the subpixels 212R, 212G, and 212B) is electricallyconnected to wirings 231, 247, 248, 249, and 250. In addition, thesubpixels 212R, 212G, and 212B are connected to respective wirings 253which are independent of one another. In this specification, forexample, the wirings 248 and 249 that are connected to the pixel 211 inthe n-th row (n is an integer greater than or equal to 1 and less thanor equal to p) are referred to as a wiring 248[n] and a wiring 249[n].For example, the wiring 253 connected to the pixel 211 in the m-thcolumn (m is an integer greater than or equal to 1 and less than orequal to q) is referred to as a wiring 253[m]. Note that in FIG. 32A,the wirings 253 connected to the subpixels 212R, 212G, and 212B in thepixel 211 in the m-th column are referred to as a wiring 253[m]R, awiring 253[m]G, and a wiring 253[m]B. The subpixels 212 are electricallyconnected to the peripheral circuit through the above wirings.

The imaging device 200 has a structure in which the subpixel 212 iselectrically connected to the subpixel 212 in an adjacent pixel 211which is provided with a color filter transmitting light in the samewavelength range as the subpixel 212, via a switch. FIG. 32B shows aconnection example of the subpixels 212: the subpixel 212 in the pixel211 arranged in an n-th row and an m-th column and the subpixel 212 inthe adjacent pixel 211 arranged in an (n+1)-th row and the m-th column.In FIG. 32B, the subpixel 212R arranged in the n-th row and the m-thcolumn and the subpixel 212R arranged in the (n+1)-th row and the m-thcolumn are connected to each other via a switch 201. The subpixel 212Garranged in the n-th row and the m-th column and the subpixel 212Garranged in the (n+1)-th row and the m-th column are connected to eachother via a switch 202. The subpixel 212B arranged in the n-th row andthe m-th column and the subpixel 212B arranged in the (n+1)-th row andthe m-th column are connected to each other via a switch 203.

The color filter used in the subpixel 212 is not limited to red (R),green (G), and blue (B) color filters, and color filters that transmitlight of cyan (C), yellow (Y), and magenta (M) may be used. By provisionof the subpixels 212 that sense light in three different wavelengthranges in one pixel 211, a full-color image can be obtained.

The pixel 211 including the subpixel 212 provided with a color filtertransmitting yellow (Y) light may be provided, in addition to thesubpixels 212 provided with the color filters transmitting red (R),green (G), and blue (B) light. The pixel 211 including the subpixel 212provided with a color filter transmitting blue (B) light may beprovided, in addition to the subpixels 212 provided with the colorfilters transmitting cyan (C), yellow (Y), and magenta (M) light. Whenthe subpixels 212 sensing light in four different wavelength ranges areprovided in one pixel 211, the reproducibility of colors of an obtainedimage can be increased.

For example, in FIG. 32A, in regard to the subpixel 212 sensing light ina red wavelength range, the subpixel 212 sensing light in a greenwavelength range, and the subpixel 212 sensing light in a bluewavelength range, the pixel number ratio (or the light receiving arearatio) thereof is not necessarily 1:1:1. For example, the Bayerarrangement in which the pixel number ratio (the light receiving arearatio) of red to green and blue is 1:2:1 may be employed. Alternatively,the pixel number ratio (the light receiving area ratio) of red to greenand blue may be 1:6:1.

Although the number of subpixels 212 provided in the pixel 211 may beone, two or more subpixels are preferably provided. For example, whentwo or more subpixels 212 sensing light in the same wavelength range areprovided, the redundancy is increased, and the reliability of theimaging device 200 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbsor reflects visible light is used as the filter, the imaging device 200that senses infrared light can be provided.

Furthermore, when a neutral density (ND) filter (dark filter) is used,output saturation which occurs when a large amount of light enters aphotoelectric conversion element (light-receiving element) can beprevented. With a combination of ND filters with different dimmingcapabilities, the dynamic range of the imaging device can be increased.

Besides the above-described filter, the pixel 211 may be provided with alens. An arrangement example of the pixel 211, a filter 254, and a lens255 is described with cross-sectional views in FIGS. 33A and 33B. Withthe lens 255, the photoelectric conversion element can receive incidentlight efficiently. Specifically, as illustrated in FIG. 33A, light 256enters a photoelectric conversion element 220 through the lens 255, thefilter 254 (filters 254R, 254G, and 254B), a pixel circuit 230, and thelike which are provided in the pixel 211.

However, as indicated by a region surrounded with dashed-dotted lines,part of the light 256 indicated by arrows might be blocked by somewirings 257. Thus, a preferable structure is that the lens 255 and thefilter 254 are provided on the photoelectric conversion element 220side, so that the photoelectric conversion element 220 can efficientlyreceive the light 256 as illustrated in FIG. 33B. When the light 256enters the photoelectric conversion element 220 from the photoelectricconversion element 220 side, the imaging device 200 with high detectionsensitivity can be provided.

As the photoelectric conversion element 220 illustrated in FIGS. 33A and33B, a photoelectric conversion element in which a p-n junction or ap-i-n junction is formed may be used.

The photoelectric conversion element 220 may be formed using a substancethat has a function of absorbing a radiation and generating electriccharges. Examples of the substance that has a function of absorbing aradiation and generating electric charges include selenium, lead iodide,mercury iodide, gallium arsenide, cadmium telluride, and cadmium zincalloy.

For example, when selenium is used for the photoelectric conversionelement 220, the photoelectric conversion element 220 can have a lightabsorption coefficient in a wide wavelength range, such as visiblelight, ultraviolet light, infrared light, X-rays, and gamma rays.

One pixel 211 included in the imaging device 200 may include thesubpixel 212 with a first filter in addition to the subpixel 212illustrated in FIGS. 32A and 32B.

Configuration Example 2 of Pixel

An example of a pixel including a transistor using silicon and atransistor using an oxide semiconductor is described below.

FIGS. 34A and 34B are each a cross-sectional view of an element includedin an imaging device.

The imaging device illustrated in FIG. 34A includes a transistor 351including silicon over a silicon substrate 300, a transistor 353 whichincludes an oxide semiconductor and is stacked over the transistor 351,and a photodiode 360 provided in the silicon substrate 300 and includingan anode 361 and a cathode 362. The transistors and the photodiode 360are electrically connected to various plugs 370 and wirings 371, 372,and 373. In addition, the anode 361 of the photodiode 360 iselectrically connected to the plug 370 through a low-resistance region363.

The imaging device includes a layer 310 including the transistor 351provided on the silicon substrate 300 and the photodiode 360 provided inthe silicon substrate 300, a layer 320 which is in contact with thelayer 310 and includes the wirings 371, a layer 330 which is in contactwith the layer 320 and includes the transistor 353 and an insulatinglayer 380, and a layer 340 which is in contact with the layer 330 andincludes the wiring 372 and the wiring 373.

Note that in the example of the cross-sectional view in FIG. 34A, alight-receiving surface of the photodiode 360 is provided on the sideopposite to a surface of the silicon substrate 300 where the transistor351 is formed. With the structure, an optical path can be obtainedwithout the influence by the transistors, wirings, and the like. Thus, apixel with a high aperture ratio can be formed. Note that thelight-receiving surface of the photodiode 360 can be the same as thesurface where the transistor 351 is formed.

In the case where a pixel is formed with the use of only transistorsusing an oxide semiconductor, the layer 310 may include the transistorusing an oxide semiconductor. Alternatively, the layer 310 may beomitted, and the pixel may include only transistors using an oxidesemiconductor.

In addition, in the cross-sectional view in FIG. 34A, the photodiode 360in the layer 310 and the transistor in the layer 330 can be formed so asto overlap with each other. Thus, the degree of integration of pixelscan be increased. In other words, the resolution of the imaging devicecan be increased.

An imaging device illustrated in FIG. 34B includes a photodiode 365 inthe layer 340 and over the transistor. In FIG. 34B, the layer 310includes the transistor 351 using silicon, the layer 320 includes thewiring 371, the layer 330 includes the transistor 353 using an oxidesemiconductor and the insulating layer 380, and the layer 340 includesthe photodiode 365. The photodiode 365 is electrically connected to thewiring 373 and a wiring 374 through the plug 370.

The element structure illustrated in FIG. 34B can increase the apertureratio.

Alternatively, a PIN diode element formed using an amorphous siliconfilm, a microcrystalline silicon film, or the like may be used as thephotodiode 365. In the photodiode 365, an n-type semiconductor 368, ani-type semiconductor 367, and a p-type semiconductor 366 are stacked inthis order. The i-type semiconductor 367 is preferably formed usingamorphous silicon. The p-type semiconductor 366 and the n-typesemiconductor 368 can each be formed using amorphous silicon,microcrystalline silicon, or the like which includes a dopant impartingthe corresponding conductivity type. The photodiode 365 in which aphotoelectric conversion layer is formed using amorphous silicon hashigh sensitivity in a visible light wavelength region, and therefore caneasily sense weak visible light.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 4

In this embodiment, circuit configuration examples to which thetransistors including oxide semiconductor layers (OS transistors)described in the above embodiment can be applied will be described withreference to FIGS. 35A to 35C, FIGS. 36A to 36C, FIGS. 37A and 37B, andFIGS. 38A and 38B.

FIG. 35A is a circuit diagram of an inverter applicable to a memory, anFPGA, a CPU, or the like. An inverter 2800 outputs a signal whose logicis inverted from the logic of a signal supplied to an input terminal INto an output terminal OUT. The inverter 2800 includes a plurality of OStransistors. A signal S_(BG) can switch electrical characteristics ofthe OS transistors.

FIG. 35B is a circuit diagram illustrating an example of the inverter2800. The inverter 2800 includes OS transistors 2810 and 2820. Theinverter 2800 can be formed using n-channel transistors and can have acircuit configuration in which all the transistors have the sameconductivity. With the circuit configuration in which all thetransistors have the same conductivity, the inverter can be formed atlower cost than an inverter formed using a complementary metal oxidesemiconductor (i.e., a CMOS inverter).

Note that the inverter 2800 including the OS transistors can be providedover a CMOS circuit including Si transistors. Since the inverter 2800can be provided so as to overlap with the CMOS circuit, no additionalarea is required for the inverter 2800, and thus, an increase in thecircuit area can be suppressed.

Each of the OS transistors 2810 and 2820 includes a first gatefunctioning as a front gate, a second gate functioning as a back gate, afirst terminal functioning as one of a source and a drain, and a secondterminal functioning as the other of the source and the drain.

The first gate of the OS transistor 2810 is connected to its secondterminal. The second gate of the OS transistor 2810 is connected to awiring that transmits the signal S_(BG). The first terminal of the OStransistor 2810 is connected to a wiring that supplies a voltage VDD.The second terminal of the OS transistor 2810 is connected to the outputterminal OUT.

The first gate of the OS transistor 2820 is connected to the inputterminal IN. The second gate of the OS transistor 2820 is connected tothe input terminal IN. The first terminal of the OS transistor 2820 isconnected to the output terminal OUT. The second terminal of the OStransistor 2820 is connected to a wiring that supplies a voltage VSS.

FIG. 35C is a timing chart illustrating the operation of the inverter2800. The timing chart in FIG. 35C illustrates changes of a signalwaveform of the input terminal IN, a signal waveform of the outputterminal OUT, a signal waveform of the signal S_(BG), and the thresholdvoltage of the OS transistor 2810.

The signal S_(BG) supplied to the second gate of the OS transistor 2810can control the threshold voltage of the OS transistor 2810.

The signal S_(BG) includes a voltage V_(BG) _(_) _(A) for shifting thethreshold voltage in the negative direction and a voltage V_(BG) _(_)_(B) for shifting the threshold voltage in the positive direction. Thethreshold voltage of the OS transistor 2810 can be shifted in thenegative direction to be a threshold voltage V_(TH) _(_) _(A) when thevoltage V_(BG) _(_) _(A) is applied to the second gate. The thresholdvoltage of the OS transistor 2810 can be shifted in the positivedirection to be a threshold voltage V_(TH) _(_) _(B) when the voltageV_(BG) _(_) _(B) is applied to the second gate.

To visualize the above description, FIG. 36A shows a V_(g)-I_(d) curve,which is one of the electrical characteristics of a transistor.

When a high voltage such as the voltage V_(BG) _(_) _(A) is applied tothe second gate, the electrical characteristics of the OS transistor2810 can be shifted to match a curve shown by a dashed line 2840 in FIG.36A. When a low voltage such as the voltage V_(BG) _(_) _(B) is appliedto the second gate, the electrical characteristics of the OS transistor2810 can be shifted to match a curve shown by a solid line 2841 in FIG.36A. As shown in FIG. 36A, switching the signal S_(BG) between thevoltage V_(BG) _(_) _(A) and the voltage V_(BG) _(_) _(B) enables thethreshold voltage of the OS transistor 2810 to be shifted in thepositive direction or the negative direction.

The shift of the threshold voltage in the positive direction toward thethreshold voltage V_(TH) _(_) _(B) can make current less likely to flowin the OS transistor 2810. FIG. 36B visualizes the state. As illustratedin FIG. 36B, a current I_(B) that flows in the OS transistor 2810 can beextremely low. Thus, when a signal supplied to the input terminal IN isat a high level and the OS transistor 2820 is on (ON), the voltage ofthe output terminal OUT can drop sharply.

Since a state in which current is less likely to flow in the OStransistor 2810 as illustrated in FIG. 36B can be obtained, a signalwaveform 2831 of the output terminal in the timing chart in FIG. 35C canbe made steep. Shoot-through current between the wiring that suppliesthe voltage VDD and the wiring that supplies the voltage VSS can be low,leading to low-power operation.

The shift of the threshold voltage in the negative direction toward thethreshold voltage V_(TH) _(_) _(A) can make current flow easily in theOS transistor 2810. FIG. 36C visualizes the state. As illustrated inFIG. 36C, a current I_(A) flowing at this time can be higher than atleast the current I_(B). Thus, when a signal supplied to the inputterminal IN is at a low level and the OS transistor 2820 is off (OFF),the voltage of the output terminal OUT can be increased sharply.

Since a state in which current is likely to flow in the OS transistor2810 as illustrated in FIG. 36C can be obtained, a signal waveform 2832of the output terminal in the timing chart in FIG. 35C can be madesteep.

Note that the threshold voltage of the OS transistor 2810 is preferablycontrolled by the signal S_(BG) before the state of the OS transistor2820 is switched, i.e., before Time T1 or T2. For example, as in FIG.35C, it is preferable that the threshold voltage of the OS transistor2810 be switched from the threshold voltage V_(TH) _(_) _(A) to thethreshold voltage V_(TH) _(_) _(B) before Time T1 at which the level ofthe signal supplied to the input terminal IN is switched to a highlevel. Moreover, as in FIG. 35C, it is preferable that the thresholdvoltage of the OS transistor 2810 be switched from the threshold voltageV_(TH) _(_) _(B) to the threshold voltage V_(TH) _(_) _(A) before TimeT2 at which the level of the signal supplied to the input terminal IN isswitched to a low level.

Although the timing chart in FIG. 35C illustrates the configuration inwhich the level of the signal S_(BG) is switched in accordance with thesignal supplied to the input terminal IN, a different configuration maybe employed in which voltage for controlling the threshold voltage isheld by the second gate of the OS transistor 2810 in a floating state,for example. FIG. 37A illustrates an example of such a circuitconfiguration.

The circuit configuration in FIG. 37A is the same as that in FIG. 35B,except that an OS transistor 2850 is added. A first terminal of the OStransistor 2850 is connected to the second gate of the OS transistor2810. A second terminal of the OS transistor 2850 is connected to awiring that supplies the voltage V_(BG) _(_) _(B) (or the voltage V_(BG)_(_) _(A)). A first gate of the OS transistor 2850 is connected to awiring that supplies a signal S_(F). A second gate of the OS transistor2850 is connected to the wiring that supplies the voltage V_(BG) _(_)_(B) (or the voltage V_(BG) _(_) _(A)).

The operation with the circuit configuration in FIG. 37A is describedwith reference to the timing chart in FIG. 37B.

The voltage for controlling the threshold voltage of the OS transistor2810 is supplied to the second gate of the OS transistor 2810 beforeTime T3 at which the level of the signal supplied to the input terminalIN is switched to a high level. The signal S_(F) is set to a high leveland the OS transistor 2850 is turned on, so that the voltage V_(BG) _(_)_(B) for controlling the threshold voltage is supplied to a node N_(BG).

The OS transistor 2850 is turned off after the voltage of the nodeN_(BG) becomes V_(BG) _(_) _(B). Since the off-state current of the OStransistor 2850 is extremely low, the voltage V_(BG) _(_) _(B) held bythe node N_(BG) can be retained while the OS transistor 2850 remainsoff. Therefore, the number of times the voltage V_(BG) _(_) _(B) issupplied to the second gate of the OS transistor 2850 can be reduced andaccordingly, the power consumption for rewriting the voltage V_(BG) _(_)_(B) can be reduced.

Although FIG. 35B and FIG. 37A each illustrate the case where thevoltage is supplied to the second gate of the OS transistor 2810 bycontrol from the outside, a different configuration may be employed inwhich voltage for controlling the threshold voltage is generated on thebasis of the signal supplied to the input terminal IN and supplied tothe second gate of the OS transistor 2810, for example. FIG. 38Aillustrates an example of such a circuit configuration.

The circuit configuration in FIG. 38A is the same as that in FIG. 35B,except that a CMOS inverter 2860 is provided between the input terminalIN and the second gate of the OS transistor 2810. An input terminal ofthe CMOS inverter 2860 is connected to the input terminal IN. An outputterminal of the CMOS inverter 2860 is connected to the second gate ofthe OS transistor 2810.

The operation with the circuit configuration in FIG. 38A is describedwith reference to a timing chart in FIG. 38B. The timing chart in FIG.38B illustrates changes of a signal waveform of the input terminal IN, asignal waveform of the output terminal OUT, an output waveform IN_B ofthe CMOS inverter 2860, and a threshold voltage of the OS transistor2810.

The output waveform IN_B which corresponds to a signal whose logic isinverted from the logic of the signal supplied to the input terminal INcan be used as a signal that controls the threshold voltage of the OStransistor 2810. Therefore, the threshold voltage of the OS transistor2810 can be controlled as described with reference to FIGS. 36A to 36C.For example, the signal supplied to the input terminal IN is at a highlevel and the OS transistor 2820 is turned on at Time T4 in FIG. 38B. Atthis time, the output waveform IN_B is at a low level. Accordingly,current can be made less likely to flow in the OS transistor 2810; thus,a drop in the voltage of the output terminal OUT can be made steep.

Moreover, the signal supplied to the input terminal IN is at a low leveland the OS transistor 2820 is turned off at Time T5 in FIG. 38B. At thistime, the output waveform IN_B is at a high level. Accordingly, currentcan easily flow in the OS transistor 2810; thus, a rise in the voltageof the output terminal OUT can be made steep.

As described above, in the configuration of the inverter including theOS transistor in this embodiment, the voltage of the back gate isswitched in accordance with the logic of the signal supplied to theinput terminal IN. In such a configuration, the threshold voltage of theOS transistor can be controlled. The control of the threshold voltage ofthe OS transistor by the signal supplied to the input terminal IN canmake a change in the voltage of the output terminal OUT steep. Moreover,shoot-through current between the wirings that supply power supplyvoltages can be reduced. Thus, power consumption can be reduced.

Embodiment 5

In this embodiment, examples of a semiconductor device which includes aplurality of circuits including OS transistors and described in theabove embodiment are described with reference to FIGS. 39A to 39E, FIGS.40A and 40B, FIGS. 41A and 41B, and FIGS. 42A to 42C.

<8. Circuit Configuration Example of Semiconductor Device>

FIG. 39A is a block diagram of a semiconductor device 900. Thesemiconductor device 900 includes a power supply circuit 901, a circuit902, a voltage generation circuit 903, a circuit 904, a voltagegeneration circuit 905, and a circuit 906.

The power supply circuit 901 is a circuit that generates a voltageV_(ORG) used as a reference. The voltage V_(ORG) is not necessarily onevoltage and can be a plurality of voltages. The voltage V_(ORG) can begenerated on the basis of a voltage V₀ supplied from the outside of thesemiconductor device 900. The semiconductor device 900 can generate thevoltage V_(ORG) on the basis of one power supply voltage supplied fromthe outside. Thus, the semiconductor device 900 can operate withoutsupply of a plurality of power supply voltages from the outside.

The circuits 902, 904, and 906 operate with different power supplyvoltages. For example, the power supply voltage of the circuit 902 is avoltage based on the voltage V_(ORG) and the voltage V_(SS)(V_(ORG)>V_(SS)), the power supply voltage of the circuit 904 is avoltage based on a voltage V_(POG) and the voltage V_(SS)(V_(POG)>V_(ORG)), and the power supply voltages of the circuit 906 arevoltages based on the voltage V_(ORG), the voltage V_(SS), and a voltageV_(NEG) (V_(ORG)>V_(SS)>V_(NEG)). When the voltage V_(SS) is set to aground potential (GND), the kinds of voltages generated by the powersupply circuit 901 can be reduced.

The voltage generation circuit 903 is a circuit that generates thevoltage V_(POG). The voltage generation circuit 903 can generate thevoltage V_(POG) on the basis of the voltage V_(ORG) supplied from thepower supply circuit 901. Thus, the semiconductor device 900 includingthe circuit 904 can operate on the basis of one power supply voltagesupplied from the outside.

The voltage generation circuit 905 is a circuit that generates thevoltage V_(NEG). The voltage generation circuit 905 can generate thevoltage V_(NEG) on the basis of the voltage V_(ORG) supplied from thepower supply circuit 901. Thus, the semiconductor device 900 includingthe circuit 906 can operate on the basis of one power supply voltagesupplied from the outside.

FIG. 39B illustrates an example of the circuit 904 that operates withthe voltage V_(POG) and FIG. 39C illustrates an example of a waveform ofa signal for operating the circuit 904.

FIG. 39B illustrates a transistor 911. A signal supplied to a gate ofthe transistor 911 is generated on the basis of, for example, thevoltage V_(POG) and the voltage V_(SS). The signal is generated on thebasis of the voltage V_(POG) to turn on the transistor 911 and on thebasis of the voltage V_(SS) to turn off the transistor 911. Asillustrated in FIG. 39C, the voltage V_(POG) is higher than the voltageV_(ORG). Thus, a source (S) and a drain (D) of the transistor 911 can beelectrically connected to each other without fail. As a result, thefrequency of malfunction of the circuit 904 can be reduced.

FIG. 39D illustrates an example of the circuit 906 that operates withthe voltage V_(NEG) and FIG. 39E illustrates an example of a waveform ofa signal for operating the circuit 906.

FIG. 39D illustrates a transistor 912 having a back gate. A signalsupplied to a gate of the transistor 912 is generated on the basis of,for example, the voltage V_(ORG) and the voltage V_(SS). The signal isgenerated on the basis of the voltage V_(ORG) to turn on the transistor912 and on the basis of the voltage V_(SS) to turn off the transistor912. A signal supplied to the back gate of the transistor 912 isgenerated on the basis of the voltage V_(NEG). As illustrated in FIG.39E, the voltage V_(NEG) is lower than the voltage V_(SS) (GND). Thus,the threshold voltage of the transistor 912 can be controlled to shiftin the positive direction. Thus, the transistor 912 can be turned offwithout fail and a current flowing between a source (S) and a drain (D)can be reduced. As a result, the frequency of malfunction of the circuit906 can be reduced and power consumption thereof can be reduced.

The voltage V_(NEG) may be directly supplied to the back gate of thetransistor 912. Alternatively, a signal supplied to the gate of thetransistor 912 may be generated on the basis of the voltage V_(ORG) andthe voltage V_(NEG) and the generated signal may also be supplied to theback gate of the transistor 912.

FIGS. 40A and 40B illustrate a modification example of FIGS. 39D and39E.

In a circuit diagram illustrated in FIG. 40A, a transistor 922 whoseon/off state can be controlled by a control circuit 921 is providedbetween the voltage generation circuit 905 and the circuit 906. Thetransistor 922 is an n-channel OS transistor. The control signal S_(BG)output from the control circuit 921 is a signal for controlling theon/off state of the transistor 922. Transistors 912A and 912B includedin the circuit 906 are OS transistors like the transistor 922.

A timing chart in FIG. 40B shows changes in a potential of the controlsignal S_(BG) and a potential of a node N_(BG). The potential of thenode N_(BG) indicates the states of potentials of back gates of thetransistors 912A and 912B. When the control signal S_(BG) is at a highlevel, the transistor 922 is turned on and the voltage of the nodeN_(BG) becomes the voltage V_(NEG). Then, when the control signal S_(BG)is at a low level, the node N_(BG) is brought into an electricallyfloating state. Since the transistor 922 is an OS transistor, itsoff-state current is small. Accordingly, even when the node N_(BG) is inan electrically floating state, the voltage V_(NEG) which has beensupplied can be held.

FIG. 41A illustrates an example of a circuit configuration applicable tothe above-described voltage generation circuit 903. The voltagegeneration circuit 903 illustrated in FIG. 41A is a five-stage chargepump including diodes D1 to D5, capacitors C1 to C5, and an inverterINV. A clock signal CLK is supplied to the capacitors C1 to C5 directlyor through the inverter INV. When a power supply voltage of the inverterINV is a voltage based on the voltage V_(ORG) and the voltage V_(SS),the voltage V_(POG) can be obtained by increasing the voltage V_(ORG) bya voltage five times a potential difference between the voltage V_(ORG)and the voltage V_(SS). Note that the forward voltage of the diodes D1to D5 is 0 V. The number of stages of the charge pump can be changed toobtain a desired voltage V_(POG).

FIG. 41B illustrates an example of a circuit configuration applicable tothe above-described voltage generation circuit 905. The voltagegeneration circuit 905 illustrated in FIG. 41B is a four-stage chargepump including the diodes D1 to D5, the capacitors C1 to C5, and theinverter INV. The clock signal CLK is supplied to the capacitors C1 toC5 directly or through the inverter INV. When a power supply voltage ofthe inverter INV is a voltage applied on the basis of the voltageV_(ORG) and the voltage V_(SS), the voltage V_(NEG) can be obtained bydecreasing the ground voltage, i.e., the voltage V_(SS) by a voltagefour times the potential difference between the voltage V_(ORG) and thevoltage V_(SS) with the application of the clock signal CLK. Note thatthe forward voltage of the diodes D1 to D5 is 0 V. The number of stagesof the charge pump can be changed to obtain a desired voltage V_(NEG).

The circuit configuration of the voltage generation circuit 903 is notlimited to the configuration in the circuit diagram illustrated in FIG.41A. Modification examples of the voltage generation circuit 903 areillustrated in FIGS. 42A to 42C. Note that further modification examplesof the voltage generation circuit 903 can be realized by changingvoltages supplied to wirings or arrangement of elements in voltagegeneration circuits 903A to 903C illustrated in FIGS. 42A to 42C

The voltage generation circuit 903A illustrated in FIG. 42A includestransistors M1 to M10, capacitors C11 to C14, and an inverter INV1. Theclock signal CLK is supplied to gates of the transistors M1 to M10directly or through the inverter INV1. By application of the clocksignal CLK, the voltage V_(POG), which has been increased to a positivevoltage having a positively quadrupled value of the voltage V_(ORG), canbe obtained. The number of stages can be changed to obtain a desiredvoltage V_(POG). In the voltage generation circuit 903A in FIG. 42A, theoff-state current of each of the transistors M1 to M10 can be small whenthe transistors M1 to M10 are OS transistors, and the leakage of chargeheld in the capacitors C11 to C14 can be reduced. Accordingly, thevoltage V_(ORG) can be efficiently increased to the voltage V_(POG).

The voltage generation circuit 903B illustrated in FIG. 42B includestransistors M11 to M14, capacitors C15 and C16, and an inverter INV2.The clock signal CLK is supplied to gates of the transistors M11 to M14directly or through the inverter INV2. By application of the clocksignal CLK, the voltage V_(POG), which has been increased to a positivevoltage having a positively doubled value of the voltage V_(ORG), can beobtained. In the voltage generation circuit 903B in FIG. 42B, off-statecurrent of each of the transistors M11 to M14 can be small when thetransistors M11 to M14 are OS transistors, and leakage of charge held inthe capacitors C15 and C16 can be suppressed. Accordingly, the voltageV_(ORG) can be efficiently increased to the voltage V_(POG).

A voltage generation circuit 903C illustrated in FIG. 42C includes aninductor I1, a transistor M15, a diode D6, and a capacitor C17. Theon/off state of the transistor M15 is controlled by a control signal EN.Owing to the control signal EN, the voltage V_(POG) increased from thevoltage V_(ORG) can be obtained. Since the voltage generation circuit903C in FIG. 42C increases the voltage using the inductor I1, thevoltage can be efficiently increased.

As described above, in any of the structures of this embodiment, avoltage required for circuits included in a semiconductor device can beinternally generated. Thus, in the semiconductor device, the number ofpower supply voltages supplied from the outside can be reduced.

Embodiment 6 <RF Tag>

In this embodiment, an RF tag that includes the transistor described inthe above embodiment or the memory device described in the aboveembodiment will be described with reference to FIG. 43.

The RF tag of this embodiment includes a memory circuit, storesnecessary data in the memory circuit, and transmits and receives datato/from the outside by using a contactless means, for example, wirelesscommunication. With these features, the RF tag can be used for anindividual authentication system in which an object or the like isrecognized by reading the individual information, for example. Note thatthe RF tag is required to have extremely high reliability in order to beused for this purpose.

A configuration of the RF tag is described with reference to FIG. 43.FIG. 43 is a block diagram illustrating a configuration example of an RFtag.

As shown in FIG. 43, an RF tag 800 includes an antenna 804 whichreceives a radio signal 803 that is transmitted from an antenna 802connected to a communication device 801 (also referred to as aninterrogator, a reader/writer, or the like). The RF tag 800 includes arectifier circuit 805, a constant voltage circuit 806, a demodulationcircuit 807, a modulation circuit 808, a logic circuit 809, a memorycircuit 810, and a ROM 811. A transistor having a rectifying functionincluded in the demodulation circuit 807 may be formed using a materialwhich enables a reverse current to be low enough, for example, an oxidesemiconductor. This can suppress the phenomenon of a rectifying functionbecoming weaker due to generation of a reverse current and preventsaturation of the output from the demodulation circuit. In other words,the input to the demodulation circuit and the output from thedemodulation circuit can have a relation closer to a linear relation.Note that data transmission methods are roughly classified into thefollowing three methods: an electromagnetic coupling method in which apair of coils is provided so as to face each other and communicates witheach other by mutual induction, an electromagnetic induction method inwhich communication is performed using an induction field, and a radiowave method in which communication is performed using a radio wave. Anyof these methods can be used in the RF tag 800 described in thisembodiment.

Next, the structure of each circuit is described. The antenna 804exchanges the radio signal 803 with the antenna 802 which is connectedto the communication device 801. The rectifier circuit 805 generates aninput potential by rectification, for example, half-wave voltage doublerrectification of an input alternating signal generated by reception of aradio signal at the antenna 804 and smoothing of the rectified signalwith a capacitor provided in a later stage. Note that a limiter circuitmay be provided on an input side or an output side of the rectifiercircuit 805. The limiter circuit controls electric power so thatelectric power which is higher than or equal to certain electric poweris not input to a circuit in a later stage if the amplitude of the inputalternating signal is high and an internal generation voltage is high.

The constant voltage circuit 806 generates a stable power supply voltagefrom an input potential and supplies it to each circuit. Note that theconstant voltage circuit 806 may include a reset signal generationcircuit. The reset signal generation circuit is a circuit whichgenerates a reset signal of the logic circuit 809 by utilizing rise ofthe stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal byenvelope detection and generates the demodulated signal. Further, themodulation circuit 808 performs modulation in accordance with data to beoutput from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. Thememory circuit 810 holds the input data and includes a row decoder, acolumn decoder, a memory region, and the like. Further, the ROM 811stores an identification number (ID) or the like and outputs it inaccordance with processing.

Note that the decision whether each circuit described above is providedor not can be made as appropriate as needed.

Here, the memory circuit described in the above embodiment can be usedfor the memory circuit 810. Since the memory circuit of one embodimentof the present invention can retain data even when not powered, thememory circuit can be favorably used for an RF tag. Furthermore, thememory circuit of one embodiment of the present invention needs power(voltage) needed for data writing significantly lower than that neededin a conventional nonvolatile memory; thus, it is possible to prevent adifference between the maximum communication range in data reading andthat in data writing. In addition, it is possible to suppressmalfunction or incorrect writing which is caused by power shortage indata writing.

Since the memory circuit of one embodiment of the present invention canbe used as a nonvolatile memory, it can also be used as the ROM 811. Inthis case, it is preferable that a manufacturer separately prepare acommand for writing data to the ROM 811 so that a user cannot rewritedata freely. Since the manufacturer gives identification numbers beforeshipment and then starts shipment of products, instead of puttingidentification numbers to all the manufactured RF tags, it is possibleto put identification numbers to only good products to be shipped. Thus,the identification numbers of the shipped products are in series andcustomer management corresponding to the shipped products is easilyperformed.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 7

In this embodiment, a CPU that includes the memory device described inthe above embodiment will be described.

FIG. 44 is a block diagram illustrating a configuration example of a CPUat least partly including the transistor described in the aboveembodiment as a component.

<Circuit Diagram of CPU>

The CPU illustrated in FIG. 44 includes, over a substrate 1190, anarithmetic logic unit (ALU) 1191, an ALU controller 1192, an instructiondecoder 1193, an interrupt controller 1194, a timing controller 1195, aregister 1196, a register controller 1197, a bus interface 1198, arewritable ROM 1199, and a ROM interface 1189. A semiconductorsubstrate, an SOI substrate, a glass substrate, or the like is used asthe substrate 1190. The ROM 1199 and the ROM interface 1189 may beprovided over a separate chip. Needless to say, the CPU in FIG. 44 isjust an example in which the configuration is simplified, and an actualCPU may have a variety of configurations depending on the application.For example, the CPU may have the following configuration: a structureincluding the CPU in FIG. 44 or an arithmetic circuit is considered asone core; a plurality of the cores are included; and the cores operatein parallel. The number of bits that the CPU can process in an internalarithmetic circuit or in a data bus can be 8, 16, 32, or 64, forexample.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 44, a memory cell is provided in theregister 1196. For the memory cell of the register 1196, the transistordescribed in Embodiment 1 can be used.

In the CPU illustrated in FIG. 44, the register controller 1197 selectsoperation of retaining data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is retained by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data retention by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data retention by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

<Memory Circuit>

FIG. 45 is an example of a circuit diagram of a memory element that canbe used as the register 1196. A memory element 1200 includes a circuit1201 in which stored data is volatile when power supply is stopped, acircuit 1202 in which stored data is nonvolatile even when power supplyis stopped, switches 1203 and 1204, a logic element 1206, a capacitor1207, and a circuit 1220 having a selecting function. The circuit 1202includes a capacitor 1208 and transistors 1209 and 1210. Note that thememory element 1200 may further include another element such as a diode,a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used asthe circuit 1202. When supply of a power supply voltage to the memoryelement 1200 is stopped, a ground potential (0 V) or a potential atwhich the transistor 1209 in the circuit 1202 is turned off continues tobe input to a gate of the transistor 1209. For example, a first gate ofthe transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213having one conductivity type (e.g., an n-channel transistor) and theswitch 1204 is a transistor 1214 having a conductivity type opposite tothe one conductivity type (e.g., a p-channel transistor). A firstterminal of the switch 1203 corresponds to one of a source and a drainof the transistor 1213, a second terminal of the switch 1203 correspondsto the other of the source and the drain of the transistor 1213, andconduction or non-conduction between the first terminal and the secondterminal of the switch 1203 (i.e., the on/off state of the transistor1213) is selected by a control signal RD input to a gate of thetransistor 1213. A first terminal of the switch 1204 corresponds to oneof a source and a drain of the transistor 1214, a second terminal of theswitch 1204 corresponds to the other of the source and the drain of thetransistor 1214, and conduction or non-conduction between the firstterminal and the second terminal of the switch 1204 (i.e., the on/offstate of the transistor 1214) is selected by the control signal RD inputto a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electricallyconnected to one of a pair of electrodes of the capacitor 1208 and agate of the transistor 1210. Here, the connection portion is referred toas a node M2. One of a source and a drain of the transistor 1210 iselectrically connected to a line which can supply a low power supplypotential (e.g., a GND line), and the other thereof is electricallyconnected to the first terminal of the switch 1203 (the one of thesource and the drain of the transistor 1213). The second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is electrically connected to the first terminal of the switch 1204(the one of the source and the drain of the transistor 1214). The secondterminal of the switch 1204 (the other of the source and the drain ofthe transistor 1214) is electrically connected to a line which cansupply a power supply potential VDD. The second terminal of the switch1203 (the other of the source and the drain of the transistor 1213), thefirst terminal of the switch 1204 (the one of the source and the drainof the transistor 1214), an input terminal of the logic element 1206,and one of a pair of electrodes of the capacitor 1207 are electricallyconnected to each other. Here, the connection portion is referred to asa node M1. The other of the pair of electrodes of the capacitor 1207 canbe supplied with a constant potential. For example, the other of thepair of electrodes of the capacitor 1207 can be supplied with a lowpower supply potential (e.g., GND) or a high power supply potential(e.g., VDD). The other of the pair of electrodes of the capacitor 1207is electrically connected to the line which can supply a low powersupply potential (e.g., a GND line). The other of the pair of electrodesof the capacitor 1208 can be supplied with a constant potential. Forexample, the other of the pair of electrodes of the capacitor 1208 canbe supplied with a low power supply potential (e.g., GND) or a highpower supply potential (e.g., VDD). The other of the pair of electrodesof the capacitor 1208 is electrically connected to the line which cansupply a low power supply potential (e.g., a GND line).

The capacitors 1207 and 1208 are not necessarily provided as long as theparasitic capacitance of the transistor, the wiring, or the like isactively utilized.

A control signal WE is input to the first gate (first gate electrode) ofthe transistor 1209. As for each of the switch 1203 and the switch 1204,a conduction state or a non-conduction state between the first terminaland the second terminal is selected by the control signal RD which isdifferent from the control signal WE. When the first terminal and thesecond terminal of one of the switches are in the conduction state, thefirst terminal and the second terminal of the other of the switches arein the non-conduction state.

Note that the transistor 1209 in FIG. 45 has a structure with a secondgate (second gate electrode: back gate). The control signal WE can beinput to the first gate and a control signal WE2 can be input to thesecond gate. The control signal WE2 is a signal having a constantpotential. As the constant potential, for example, a ground potentialGND or a potential lower than a source potential of the transistor 1209is selected. The control signal WE2 is a potential signal forcontrolling the threshold voltage of the transistor 1209, and a currentwhen a gate voltage VG is 0 V can be further reduced. The control signalWE2 may be a signal having the same potential as the control signal WE.Note that as the transistor 1209, a transistor without a second gate maybe used.

A signal corresponding to data retained in the circuit 1201 is input tothe other of the source and the drain of the transistor 1209. FIG. 44illustrates an example in which a signal output from the circuit 1201 isinput to the other of the source and the drain of the transistor 1209.The logic value of a signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) is inverted by the logic element 1206, and the inverted signal isinput to the circuit 1201 through the circuit 1220.

In the example of FIG. 45, a signal output from the second terminal ofthe switch 1203 (the other of the source and the drain of the transistor1213) is input to the circuit 1201 through the logic element 1206 andthe circuit 1220; however, one embodiment of the present invention isnot limited thereto. The signal output from the second terminal of theswitch 1203 (the other of the source and the drain of the transistor1213) may be input to the circuit 1201 without its logic value beinginverted. For example, in the case where the circuit 1201 includes anode in which a signal obtained by inversion of the logic value of asignal input from the input terminal is retained, the signal output fromthe second terminal of the switch 1203 (the other of the source and thedrain of the transistor 1213) can be input to the node.

In FIG. 45, the transistors included in the memory element 1200 exceptfor the transistor 1209 can each be a transistor in which a channel isformed in a layer formed using a semiconductor other than an oxidesemiconductor or in the substrate 1190. For example, the transistor canbe a transistor whose channel is formed in a silicon layer or a siliconsubstrate. Alternatively, all the transistors in the memory element 1200may be a transistor in which a channel is formed in an oxidesemiconductor. Further alternatively, in the memory element 1200, atransistor in which a channel is formed in an oxide semiconductor can beincluded besides the transistor 1209, and a transistor in which achannel is formed in a layer including a semiconductor other than anoxide semiconductor or the substrate 1190 can be used for the rest ofthe transistors.

As the circuit 1201 in FIG. 45, for example, a flip-flop circuit can beused. As the logic element 1206, for example, an inverter or a clockedinverter can be used.

In a period during which the memory element 1200 is not supplied withthe power supply voltage, the semiconductor device of one embodiment ofthe present invention can retain data stored in the circuit 1201 by thecapacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in anoxide semiconductor is extremely low. For example, the off-state currentof a transistor in which a channel is formed in an oxide semiconductoris significantly lower than that of a transistor in which a channel isformed in silicon having crystallinity. Thus, when the transistor isused as the transistor 1209, a signal held in the capacitor 1208 isretained for a long time also in a period during which the power supplyvoltage is not supplied to the memory element 1200. The memory element1200 can accordingly retain the stored content (data) also in a periodduring which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operationwith the switches 1203 and 1204, it is possible to shorten the timerequired for the circuit 1201 to retain original data again after thesupply of the power supply voltage is restarted.

In the circuit 1202, a signal retained by the capacitor 1208 is input tothe gate of the transistor 1210. Therefore, after supply of the powersupply voltage to the memory element 1200 is restarted, the state (theon state or the off state) of the transistor 1210 is determined inaccordance with the signal retained by the capacitor 1208 and can beread from the circuit 1202. Consequently, an original signal can beaccurately read even when a potential corresponding to the signalretained by the capacitor 1208 varies to some degree.

By applying the above-described memory element 1200 to a memory devicesuch as a register or a cache memory included in a processor, data inthe memory device can be prevented from being lost owing to the stop ofthe supply of the power supply voltage. Furthermore, shortly after thesupply of the power supply voltage is restarted, the memory device canbe returned to the same state as that before the power supply isstopped. Therefore, the power supply can be stopped even for a shorttime in the processor or one or a plurality of logic circuits includedin the processor, resulting in lower power consumption.

Although the memory element 1200 is used in a CPU in this embodiment,the memory element 1200 can also be used in an LSI such as a digitalsignal processor (DSP), a custom LSI, or a programmable logic device(PLD), and a radio frequency (RF) tag.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 8

In this embodiment, configuration examples of a display device using atransistor of one embodiment of the present invention will be described.

<Circuit Configuration Example of Display Device>

FIG. 46A is a top view of the display device of one embodiment of thepresent invention. FIG. 46B is a circuit diagram illustrating a pixelcircuit that can be used in the case where a liquid crystal element isused in a pixel in the display device of one embodiment of the presentinvention. FIG. 46C is a circuit diagram illustrating a pixel circuitthat can be used in the case where an organic EL element is used in apixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance withEmbodiment 1. The transistor can be easily formed as an n-channeltransistor, and thus part of a driver circuit that can be formed usingan n-channel transistor can be formed over the same substrate as thetransistor of the pixel portion. With the use of the transistordescribed in the above embodiment for the pixel portion or the drivercircuit in this manner, a highly reliable display device can beprovided.

FIG. 46A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 701, a first scan line driver circuit702, a second scan line driver circuit 703, and a signal line drivercircuit 704 are formed over a substrate 700 of the display device. Inthe pixel portion 701, a plurality of signal lines extended from thesignal line driver circuit 704 are arranged and a plurality of scanlines extended from the first scan line driver circuit 702 and thesecond scan line driver circuit 703 are arranged. Note that pixels whichinclude display elements are provided in a matrix in respective regionswhere the scan lines and the signal lines intersect with each other. Thesubstrate 700 of the display device is connected to a timing controlcircuit (also referred to as a controller or a controller IC) through aconnection portion such as a flexible printed circuit (FPC).

In FIG. 46A, the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 areformed over the substrate 700 where the pixel portion 701 is formed.Accordingly, the number of components which are provided outside, suchas a driver circuit, can be reduced, so that a reduction in cost can beachieved. Furthermore, if the driver circuit is provided outside thesubstrate 700, wirings would need to be extended and the number ofwiring connections would increase. When the driver circuit is providedover the substrate 700, the number of wiring connections can be reduced.Consequently, an improvement in reliability or yield can be achieved.One or more of the first scan line driver circuit 702, the second scanline driver circuit 703, and the signal line driver circuit 704 may bemounted on the substrate 700 or provided outside the substrate 700.

<Liquid Crystal Display Device>

FIG. 46B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrode layers. The pixel electrodelayers are connected to different transistors, and the transistors canbe driven with different gate signals. Accordingly, signals applied toindividual pixel electrode layers in a multi-domain pixel can becontrolled independently.

A scan line 712 of a transistor 716 and a scan line 713 of a transistor717 are separated so that different gate signals can be suppliedthereto. In contrast, a signal line 714 is shared by the transistors 716and 717. The transistor described in Embodiment 1 can be used asappropriate as each of the transistors 716 and 717. Thus, a highlyreliable liquid crystal display device can be provided.

A first pixel electrode layer is electrically connected to thetransistor 716 and a second pixel electrode layer is electricallyconnected to the transistor 717. The first pixel electrode layer and thesecond pixel electrode layer are separated. There is no particularlimitation on the shapes of the first pixel electrode layer and thesecond pixel electrode layer. For example, the first pixel electrodelayer may have a V-like shape.

A gate electrode of the transistor 716 is connected to the scan line712, and a gate electrode of the transistor 717 is connected to the scanline 713. When different gate signals are supplied to the scan line 712and the scan line 713, operation timings of the transistors 716 and 717can be varied. As a result, alignment of liquid crystals can becontrolled.

Further, a storage capacitor may be formed using a capacitor wiring 710,a gate insulating layer functioning as a dielectric, and a capacitorelectrode electrically connected to the first pixel electrode layer orthe second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and asecond liquid crystal element 719. The first liquid crystal element 718includes the first pixel electrode layer, a counter electrode layer, anda liquid crystal layer therebetween. The second liquid crystal element719 includes the second pixel electrode layer, a counter electrodelayer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited tothat illustrated in FIG. 46B. For example, a switch, a resistor, acapacitor, a transistor, a sensor, a logic circuit, or the like may beadded to the pixel circuit illustrated in FIG. 46B.

FIGS. 47A and 47B are examples of a top view and a cross-sectional viewof a liquid crystal display device. Note that FIG. 47A illustrates atypical structure including a display device 20, a display region 21, aperipheral circuit 22, and flexible printed circuits (FPCs) 42. Thedisplay device illustrated in FIGS. 47A and 47B uses a reflective liquidcrystal element.

FIG. 47B is a cross-sectional view taken along dashed lines A-A′, B-B′,C-C′, and D-D′ in FIG. 47A. The cross section taken along dashed lineA-A′ illustrates the peripheral circuit portion, the cross section takenalong dashed line B-B′ illustrates the display region, and the crosssections taken along dashed line C-C′ and dashed line D-D′ illustrateportions connected to the FPCs.

The display device 20 using the liquid crystal element includes thefollowing in addition to transistors 50 and 52 (the transistor 10described in Embodiment 1): the conductive layer 165, a conductive layer197, an insulating layer 420, a liquid crystal layer 490, a liquidcrystal element 80, capacitors 60 and 62, an insulating layer 430, aspacer 440, a coloring layer 460, a bonding layer 470, a conductivelayer 480, a light-shielding layer 418, a substrate 400, bonding layers473, 474, 475, and 476, polarizing plates 103 and 403, protectivesubstrates 105 and 402, and an anisotropic conductive layer 510.

<Organic EL Display Device>

FIG. 46C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using an organic ELelement is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodes andholes are injected from the other of the pair of electrodes, into alayer containing a light-emitting organic compound; thus, current flows.The electrons and holes are recombined, and thus, the light-emittingorganic compound is excited. The light-emitting organic compound returnsto a ground state from the excited state, thereby emitting light. Owingto such a mechanism, this light-emitting element is referred to as acurrent-excitation light-emitting element.

FIG. 46C illustrates an applicable example of a pixel circuit. Here, onepixel includes two n-channel transistors. Furthermore, digital timegrayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving are described.

A pixel 720 includes a switching transistor 721, a driver transistor722, a light-emitting element 724, and a capacitor 723. A gate electrodelayer of the switching transistor 721 is connected to a scan line 726, afirst electrode (one of a source electrode layer and a drain electrodelayer) of the switching transistor 721 is connected to a signal line725, and a second electrode (the other of the source electrode layer andthe drain electrode layer) of the switching transistor 721 is connectedto a gate electrode layer of the driver transistor 722. The gateelectrode layer of the driver transistor 722 is connected to a powersupply line 727 through the capacitor 723, a first electrode of thedriver transistor 722 is connected to the power supply line 727, and asecond electrode of the driver transistor 722 is connected to a firstelectrode (a pixel electrode) of the light-emitting element 724. Asecond electrode of the light-emitting element 724 corresponds to acommon electrode 728. The common electrode 728 is electrically connectedto a common potential line formed over the same substrate as the commonelectrode 728.

As the switching transistor 721 and the driver transistor 722, thetransistor described in Embodiment 1 can be used as appropriate. In thismanner, a highly reliable organic EL display device can be provided.

The potential of the second electrode (the common electrode 728) of thelight-emitting element 724 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 727. For example, thelow power supply potential can be GND, 0 V, or the like. The high powersupply potential and the low power supply potential are set to be higherthan or equal to the forward threshold voltage of the light-emittingelement 724, and the difference between the potentials is applied to thelight-emitting element 724, whereby current is supplied to thelight-emitting element 724, leading to light emission. The forwardvoltage of the light-emitting element 724 refers to a voltage at which adesired luminance is obtained, and includes at least a forward thresholdvoltage.

Note that gate capacitance of the driver transistor 722 may be used as asubstitute for the capacitor 723, so that the capacitor 723 can beomitted.

Next, a signal input to the driver transistor 722 is described. In thecase of a voltage-input voltage driving method, a video signal forsufficiently turning on or off the driver transistor 722 is input to thedriver transistor 722. In order for the driver transistor 722 to operatein a linear region, voltage higher than the voltage of the power supplyline 727 is applied to the gate electrode layer of the driver transistor722. Note that voltage higher than or equal to voltage which is the sumof power supply line voltage and the threshold voltage V_(th) of thedriver transistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage higherthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 724 and the threshold voltage V_(th) of thedriver transistor 722 is applied to the gate electrode layer of thedriver transistor 722. A video signal by which the driver transistor 722is operated in a saturation region is input, so that current is suppliedto the light-emitting element 724. In order for the driver transistor722 to operate in a saturation region, the potential of the power supplyline 727 is set higher than the gate potential of the driver transistor722. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 724 in accordance with the videosignal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the presentinvention is not limited to that illustrated in FIG. 46C. For example, aswitch, a resistor, a capacitor, a sensor, a transistor, a logiccircuit, or the like may be added to the pixel circuit illustrated inFIG. 46C.

In the case where the transistor described in the above embodiment isused for the circuit illustrated in FIGS. 46A to 46C, the sourceelectrode (the first electrode) is electrically connected to the lowpotential side and the drain electrode (the second electrode) iselectrically connected to the high potential side. Furthermore, thepotential of the first gate electrode may be controlled by a controlcircuit or the like, and the potential described above as an example,for example, a potential lower than the potential applied to the sourceelectrode, may be input to the second gate electrode through a wiringthat is not illustrated.

FIGS. 48A and 48B are examples of a top view and a cross-sectional viewof a display device using a light-emitting element. Note that FIG. 48Aillustrates a typical structure including a display device 24, thedisplay region 21, the peripheral circuit 22, and the flexible printedcircuit (FPCs) 42.

FIG. 48B is a cross-sectional view taken along dashed lines A-A′, B-B′,and C-C′ in FIG. 48A. The cross section taken along dashed line A-A′illustrates the peripheral circuit portion, the cross section takenalong dashed line B-B′ illustrates the display region, and the crosssections taken along dashed line C-C′ and dashed line D-D′ illustrateportions connected to the FPCs.

The display device 24 using the light-emitting element includes thefollowing in addition to the transistors 50 and 52 (the transistor 10described in Embodiment 1): the insulating layer 420, the conductivelayer 197, a conductive layer 410, an optical adjustment layer 530, anEL layer 450, a conductive layer 415, a light-emitting element 70, thecapacitors 60 and 62, the insulating layer 430, the spacer 440, thecoloring layer 460, the bonding layer 470, a partition wall 445, thelight-shielding layer 418, the substrate 400, and the anisotropicconductive layer 510.

In this specification, a display element, a display device which is adevice including a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ a variety of modes or can include a variety ofelements, for example. A display element, a display device, alight-emitting element, or a light-emitting device include at least oneof the following, for example: an EL (electroluminescent) element (e.g.,an EL element including organic and inorganic materials, an organic ELelement, or an inorganic EL element), an LED (e.g., a white LED, a redLED, a green LED, or a blue LED), a quantum dot, a transistor (atransistor which emits light depending on current), an electron emitter,a liquid crystal element, electronic ink, an electrophoretic element, agrating light valve (GLV), a plasma display panel (PDP), micro electromechanical systems (MEMS), a digital micromirror device (DMD), a digitalmicro shutter (DMS), MIRASOL (registered trademark), an interferometricmodulator (IMOD) element, an electrowetting element, a piezoelectricceramic display, and a display element using a carbon nanotube. Otherthan the above, display media whose contrast, luminance, reflectivity,transmittance, or the like is changed by electric or electromagneticaction may be included. Note that examples of display devices having ELelements include an EL display. Examples of display devices includingelectron emitters include a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Examples of display devices including electronic inkor electrophoretic elements include electronic paper.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 9

In this embodiment, a display module using a semiconductor device of oneembodiment of the present invention will be described with reference toFIG. 49.

<Display Module>

In a display module 6000 in FIG. 49, a touch panel 6004 connected to anFPC 6003, a display panel 6006 connected to an FPC 6005, a backlightunit 6007, a frame 6009, a printed circuit board 6010, and a battery6011 are provided between an upper cover 6001 and a lower cover 6002.Note that the backlight unit 6007, the battery 6011, the touch panel6004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 6006 and an integratedcircuit mounted on a printed circuit board.

The shapes and sizes of the upper cover 6001 and the lower cover 6002can be changed as appropriate in accordance with the sizes of the touchpanel 6004 and the display panel 6006.

The touch panel 6004 can be a resistive touch panel or a capacitivetouch panel and may be formed to overlap with the display panel 6006. Acounter substrate (sealing substrate) of the display panel 6006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 6006 so that an optical touch panel function is added.An electrode for a touch sensor may be provided in each pixel of thedisplay panel 6006 so that a capacitive touch panel function is added.

The backlight unit 6007 includes a light source 6008. The light source6008 may be provided at an end portion of the backlight unit 6007 and alight diffusing plate may be used.

The frame 6009 protects the display panel 6006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated fromthe printed circuit board 6010. The frame 6009 may function as aradiator plate.

The printed circuit board 6010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 6011 provided separatelymay be used. Note that the battery 6011 is not necessary in the casewhere a commercial power source is used.

The display module 6000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 10

In this embodiment, application examples of the semiconductor device ofone embodiment of the present invention will be described.

<Package Using Lead Frame Interposer>

FIG. 50A is a perspective view illustrating a cross-sectional structureof a package using a lead frame interposer. In the package illustratedin FIG. 50A, a chip 1751 corresponding to the semiconductor device ofone embodiment of the present invention is connected to a terminal 1752over an interposer 1750 by wire bonding. The terminal 1752 is placed ona surface of the interposer 1750 on which the chip 1751 is mounted. Thechip 1751 may be sealed by a mold resin 1753, in which case the chip1751 is sealed such that part of each of the terminals 1752 is exposed.

FIG. 50B illustrates the structure of a module of an electronic device(mobile phone) in which a package is mounted on a circuit board. In themodule of the mobile phone in FIG. 50B, a package 1802 and a battery1804 are mounted on a printed wiring board 1801. The printed wiringboard 1801 is mounted on a panel 1800 including a display element by anFPC 1803.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Embodiment 11

In this embodiment, electronic devices and lighting devices ofembodiments of the present invention will be described with reference todrawings.

<Electronic Device>

Electronic devices and lighting devices can be manufactured using thesemiconductor device of one embodiment of the present invention. Inaddition, highly reliable electronic devices and lighting devices can bemanufactured using the semiconductor device of one embodiment of thepresent invention. Furthermore, electronic devices and lighting devicesincluding touch sensors with improved detection sensitivity can befabricated using the semiconductor device of one embodiment of thepresent invention.

Examples of electronic devices are television devices (also referred toas TVs or television receivers), monitors for computers and the like,cameras such as digital cameras and digital video cameras, digital photoframes, cellular phones (also referred to as mobile phones or portabletelephone devices), portable game machines, portable informationterminals, audio playback devices, large game machines such as pin-ballmachines, and the like.

In the case of having flexibility, the electronic device or lightingdevice of one embodiment of the present invention can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

Furthermore, the electronic device of one embodiment of the presentinvention may include a secondary battery. It is preferable that thesecondary battery be capable of being charged by non-contact powertransmission.

Examples of the secondary battery include a lithium ion secondarybattery such as a lithium polymer battery using a gel electrolyte(lithium ion polymer battery), a lithium-ion battery, a nickel-hydridebattery, a nickel-cadmium battery, an organic radical battery, a leadstorage battery, an air secondary battery, a nickel-zinc battery, and asilver-zinc battery.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display an image, data, or the like on a displayportion. When the electronic device includes a secondary battery, theantenna may be used for non-contact power transmission.

FIG. 51A illustrates a portable game machine including a housing 7101, ahousing 7102, display portions 7103 and 7104, a microphone 7105,speakers 7106, an operation key 7107, a stylus 7108, and the like. Thesemiconductor device of one embodiment of the present invention can beused for an integrated circuit, a CPU, or the like incorporated in thehousing 7101. When a normally-off CPU is used as the CPU, powerconsumption can be reduced, allowing a user to enjoy playing a game forlonger than before. When the light-emitting device of one embodiment ofthe present invention is used as the display portion 7103 or 7104, it ispossible to provide a user-friendly portable game machine with qualitythat hardly deteriorates. Although the portable game machine illustratedin FIG. 51A includes two display portions, the display portions 7103 and7104, the number of display portions included in the portable gamemachine is not limited to two.

FIG. 51B illustrates a smart watch including a housing 7302, a displayportion 7304, operation buttons 7311 and 7312, a connection terminal7313, a band 7321, a clasp 7322, and the like. The semiconductor deviceof one embodiment of the present invention can be used for a memory, aCPU, or the like incorporated in the housing 7302. Note that when thedisplay is a reflective liquid crystal panel and the CPU is anormally-off CPU in FIG. 51B, power consumption can be reduced, leadingto a reduction in the number of times of daily charging.

FIG. 51C illustrates a portable information terminal including a displayportion 7502 incorporated in a housing 7501, operation buttons 7503, anexternal connection port 7504, a speaker 7505, a microphone 7506, adisplay portion 7502, and the like. The semiconductor device of oneembodiment of the present invention can be used for a mobile memory, aCPU, or the like incorporated in the housing 7501. Note that when anormally-off CPU is used, the number of times of charging can bereduced. The display portion 7502 is small- or medium-sized but canperform full high vision, 4K or 8K display because it has greatly highresolution; therefore, a significantly clear image can be obtained.

FIG. 51D illustrates a video camera including a first housing 7701, asecond housing 7702, a display portion 7703, operation keys 7704, a lens7705, a joint 7706, and the like. The operation keys 7704 and the lens7705 are provided for the first housing 7701, and the display portion7703 is provided for the second housing 7702. The first housing 7701 andthe second housing 7702 are connected to each other with the joint 7706,and the angle between the first housing 7701 and the second housing 7702can be changed with the joint 7706. Images displayed on the displayportion 7703 may be switched in accordance with the angle at the joint7706 between the first housing 7701 and the second housing 7702. Theimaging device of one embodiment of the present invention can be used ina portion corresponding to a focus of the lens 7705. The semiconductordevice of one embodiment of the present invention can be used for anintegrated circuit, a CPU, or the like incorporated in the first housing7701.

FIG. 51E illustrates a digital signage including a display portion 7902provided on a utility pole 7901. The semiconductor device of oneembodiment of the present invention can be used for a display panel ofthe display portion 7902 and an incorporated control circuit.

FIG. 52A illustrates a notebook personal computer including a housing8121, a display portion 8122, a keyboard 8123, a pointing device 8124,and the like. The semiconductor device of one embodiment of the presentinvention can be used for a CPU, a memory, or the like incorporated inthe housing 8121. Note that the display portion 8122 is small- ormedium-sized but can perform 8K display because it has greatly highresolution; therefore, a significantly clear image can be obtained.

FIG. 52B is an external view of an automobile 9700. FIG. 52C illustratesa driver's seat of the automobile 9700. The automobile 9700 includes acar body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like.The semiconductor device of one embodiment of the present invention canbe used in a display portion and a control integrated circuit of theautomobile 9700. For example, the semiconductor device of one embodimentof the present invention can be used in display portions 9710 to 9715illustrated in FIG. 52C.

The display portions 9710 and 9711 are display devices or input/outputdevices provided in an automobile windshield. The display device orinput/output device of one embodiment of the present invention can be asee-through display device or input/output device, through which theopposite side can be seen, by using a light-transmitting conductivematerial for its electrodes. Such a see-through display device orinput/output device does not hinder driver's vision during the drivingof the automobile 9700. Therefore, the display device or input/outputdevice of one embodiment of the present invention can be provided in thewindshield of the automobile 9700. Note that in the case where atransistor or the like for driving the display device or input/outputdevice is provided in the display device or input/output device, atransistor having light-transmitting properties, such as an organictransistor using an organic semiconductor material or a transistor usingan oxide semiconductor, is preferably used.

The display portion 9712 is a display device provided on a pillarportion. For example, the display portion 9712 can compensate for theview hindered by the pillar portion by showing an image taken by animaging unit provided on the car body. The display portion 9713 is adisplay device provided on a dashboard portion. For example, the displayportion 9713 can compensate for the view hindered by the dashboardportion by showing an image taken by an imaging unit provided on the carbody. That is, showing an image taken by an imaging unit provided on theoutside of the car body leads to elimination of blind areas andenhancement of safety. In addition, showing an image so as to compensatefor the area which a driver cannot see makes it possible for the driverto confirm safety easily and comfortably.

FIG. 52D illustrates the inside of a car in which a bench seat is usedas a driver seat and a front passenger seat. A display portion 9721 is adisplay device or an input/output device provided in a door portion. Forexample, the display portion 9721 can compensate for the view hinderedby the door portion by showing an image taken by an imaging unitprovided on the car body. A display portion 9722 is a display deviceprovided in a steering wheel. A display portion 9723 is a display deviceprovided in the middle of a seating face of the bench seat. Note thatthe display device can be used as a seat heater by providing the displaydevice on the seating face or backrest and by using heat generated bythe display device as a heat source.

The display portions 9714, 9715, and 9722 can display a variety of kindsof information such as navigation data, a speedometer, a tachometer, amileage, a fuel meter, a gearshift indicator, and air-condition setting.The content, layout, or the like of the display on the display portionscan be changed freely by a user as appropriate. The information listedabove can also be displayed on the display portions 9710 to 9713, 9721,and 9723. The display portions 9710 to 9715 and 9721 to 9723 can also beused as lighting devices. The display portions 9710 to 9715 and 9721 to9723 can also be used as heating devices.

FIG. 53A is an external view of a camera 8000. The camera 8000 includesa housing 8001, a display portion 8002, an operation button 8003, ashutter button 8004, a connection portion 8005, and the like. A lens8006 can be put on the camera 8000.

The connection portion 8005 includes an electrode to connect a finder8100, which is described below, a stroboscope, or the like.

Although the lens 8006 of the camera 8000 here is detachable from thehousing 8001 for replacement, the lens 8006 may be included in ahousing.

Images can be taken at the press of the shutter button 8004. Inaddition, images can be taken at the touch of the display portion 8002which serves as a touch panel.

The display device or input/output device of one embodiment of thepresent invention can be used in the display portion 8002.

FIG. 53B shows the camera 8000 with the finder 8100 connected.

The finder 8100 includes a housing 8101, a display portion 8102, abutton 8103, and the like.

The housing 8101 includes a connection portion for engagement with theconnection portion 8005 of the camera 8000 so that the finder 8100 canbe connected to the camera 8000. The connection portion includes anelectrode, and an image or the like received from the camera 8000through the electrode can be displayed on the display portion 8102.

The button 8103 has a function of a power button, and the displayportion 8102 can be turned on and off with the button 8103.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit and an image sensor included in thehousing 8101.

Although the camera 8000 and the finder 8100 are separate and detachableelectronic devices in FIGS. 53A and 53B, the housing 8001 of the camera8000 may include a finder having the display device or input/outputdevice of one embodiment of the present invention.

FIG. 53C is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. The mounting portion 8201 includes a battery 8206.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 8204. The movement of the eyeball and the eyelid of auser is captured by a camera in the main body 8203 and then coordinatesof the points the user looks at are calculated using the captured datato utilize the eye point of the user as an input means.

The mounting portion 8201 may include a plurality of electrodes so as tobe in contact with the user. The main body 8203 may be configured tosense current flowing through the electrodes with the movement of theuser's eyeball to recognize the direction of his or her eyes. The mainbody 8203 may be configured to sense current flowing through theelectrodes to monitor the user's pulse. The mounting portion 8201 mayinclude sensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor, so that the user's biological information can bedisplayed on the display portion 8204. The main body 8203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 8204 in synchronization withthe movement of the user's head or the like.

The semiconductor device of one embodiment of the present invention canbe used for an integrated circuit included in the main body 8203.

At least part of this embodiment can be implemented in combination withany of the embodiments described in this specification as appropriate.

Embodiment 12

In this embodiment, application examples of an RF tag using thesemiconductor device of one embodiment of the present invention aredescribed with reference to FIGS. 54A to 54F.

<Application Examples of RF Tag>

The RF tag is widely used and can be provided for, for example, productssuch as bills, coins, securities, bearer bonds, documents (e.g.,driver's licenses or resident's cards, see FIG. 54A), vehicles (e.g.,bicycles, see FIG. 54B), packaging containers (e.g., wrapping paper orbottles, see FIG. 54C), recording media (e.g., DVD or video tapes, seeFIG. 54D), personal belongings (e.g., bags or glasses), foods, plants,animals, human bodies, clothing, household goods, medical supplies suchas medicine and chemicals, and electronic devices (e.g., liquid crystaldisplay devices, EL display devices, television sets, or mobile phones),or tags on products (see FIGS. 54E and 54F).

An RF tag 4000 of one embodiment of the present invention is fixed to aproduct by being attached to a surface thereof or embedded therein. Forexample, the RF tag 4000 is fixed to each product by being embedded inpaper of a book, or embedded in an organic resin of a package. Since theRF tag 4000 of one embodiment of the present invention can be reduced insize, thickness, and weight, it can be fixed to a product withoutspoiling the design of the product. Furthermore, bills, coins,securities, bearer bonds, documents, or the like can have anidentification function by being provided with the RF tag 4000 of oneembodiment of the present invention, and the identification function canbe utilized to prevent counterfeiting. Moreover, the efficiency of asystem such as an inspection system can be improved by providing the RFtag of one embodiment of the present invention for packaging containers,recording media, personal belongings, foods, clothing, household goods,electronic devices, or the like. Vehicles can also have higher securityagainst theft or the like by being provided with the RF tag of oneembodiment of the present invention.

As described above, by using the RF tag including the semiconductordevice of one embodiment of the present invention for each applicationdescribed in this embodiment, power for operation such as writing orreading of data can be reduced, which results in an increase in themaximum communication distance. Moreover, data can be held for anextremely long period even in the state where power is not supplied;thus, the RF tag can be favorably used for application in which data isnot frequently written or read.

Note that this embodiment can be combined as appropriate with any of theother embodiments and an example in this specification.

Example 1

In this example, a sample A and a sample B were fabricated using oneembodiment of the present invention. Cross sections of the samples wereobserved and elementary analyses thereof were performed. The results aredescribed.

FIG. 56 is a schematic cross-sectional view of the evaluation samples.The sample A and the sample B each include the substrate 100, theinsulating layer 110, the oxide semiconductor layer 122, the conductivelayer 166, the conductive layer 168, and an antioxidant film 178.

A silicon substrate was used as the substrate 100.

As the insulating layer 110, a 100-nm-thick silicon oxide film wasformed by a thermal oxidation method. The insulating layer 110 wasformed by thermal oxidation treatment at 950° C. in an oxygen atmospherecontaining 3% chlorine.

The oxide semiconductor layer 122 was deposited to a thickness of 50 nmby a sputtering method using a target of In:Ga:Zn=4:2:4.1. Thedeposition conditions were as follows: the pressure in a chamber was 0.7Pa; a DC power source was used and the power was 0.5 kW; the sputteringgas flow rates of an Ar gas and an oxygen gas were 30 sccm and 15 sccm,respectively; the distance between the sample and the target was 60 mm;and the substrate heating temperature was 200° C.

As the conductive layer 166, a 10-nm-thick cobalt film was formed by asputtering method. The deposition conditions were as follows: thepressure in a chamber was 0.8 Pa; a DC power source was used and thepower was 0.5 kW; the flow rate of an Ar gas as a sputtering gas was 50sccm; the distance between the sample and the target was 150 mm; and thesubstrate heating temperature was room temperature.

As the conductive layer 168, a 20-nm-thick titanium film was depositedby a sputtering method. The deposition conditions were as follows: thepressure in a chamber was 0.1 Pa; a DC power source was used and thepower was 12 kW; the flow rate of an Ar gas as a sputtering gas was 20sccm; the distance between the sample and the target was 400 mm; and thesubstrate heating temperature was room temperature.

As the antioxidant film 178, a 20-nm-thick titanium nitride film wasdeposited by a sputtering method.

Next, heat treatment was performed on the samples. The heat treatmentwas performed using an RTA apparatus. The heat treatment was performedin two conditions. The sample A was subjected to a heat treatment at300° C. in a nitrogen atmosphere for 60 seconds; and the sample B wassubjected to a heat treatment at 500° C. in a nitrogen atmosphere for 60seconds.

The cross sections of the samples were observed with a scanningtransmittance electron microscope (STEM). As an apparatus forobservation, HD-2300 manufactured by Hitachi High-TechnologiesCorporation was used. Elemental analysis was performed using energydispersive X-ray spectroscopy (EDX). As an analyzer, Genesis 4000 R-TEMseries manufactured by EDAX Inc. was used.

FIGS. 57A and 57B each show an image in which an observation photographof the cross-section of the sample after the heat treatment overlapswith a line mapping analysis result thereof by EDX. In the line mappinganalysis results, a portion difficult to see is indicated by dottedlines.

FIG. 57A is the analysis result of the sample A. FIG. 57A shows that thediffusion of elements in layers is not observed when the heat treatmentis performed at 300° C. for 60 seconds. FIG. 57B is the analysis resultof the sample B. In FIG. 57B, however, by performing heat treatment at500° C. for 60 seconds, indium and gallium are diffused in the directionin which titanium exists and cobalt is diffused in the direction inwhich indium, gallium, and zinc which are constituent elements of theoxide semiconductor layer 122 exist. In addition, FIG. 57B alsoindicates that titanium which is a constituent element of the conductivelayer 168 is diffused.

In the case where titanium was not used in the conductive layer 168,cobalt, indium, and zinc were not diffused even when heat treatment at500° C. for 60 seconds was performed, although it is not shown in thedrawing.

Therefore, it is found that one embodiment of the present inventionpromotes diffusion of constituent elements of the oxide semiconductorlayer 122, the conductive layer 166, and the conductive layer 168,whereby an alloy is formed.

This application is based on Japanese Patent Application serial no.2015-153217 filed with Japan Patent Office on Aug. 3, 2015, the entirecontents of which are hereby incorporated by reference.

1-8. (canceled)
 9. A semiconductor device comprising: a first insulatinglayer over a substrate; a first oxide insulating layer over the firstinsulating layer; an oxide semiconductor layer over the first oxideinsulating layer; a second oxide insulating layer over the oxidesemiconductor layer; a gate insulating layer over the second oxideinsulating layer; a gate electrode layer over the gate insulating layer;and a sidewall insulating layer over the oxide semiconductor layer,wherein a bottom surface of the oxide semiconductor layer is locatedabove a bottom surface of the gate electrode layer in a cross section ina channel width direction, wherein a top surface and a side surface ofthe oxide semiconductor layer each comprise a region facing the gateelectrode layer, wherein the sidewall insulating layer comprises aregion in contact with a side surface of the gate insulating layer,wherein the oxide semiconductor layer comprises first to third regions,wherein the first region comprises a region overlapping with the gateelectrode layer, wherein the second region comprises a region which isbetween the first region and the third region and overlaps with thesidewall insulating layer, wherein the second region comprises a regionwith lower resistance than the first region, wherein the third regioncomprises a region with lower resistance than the second region, andwherein the second region and the third region each comprise any one ofelements of cobalt, nickel, vanadium, and palladium, and any one ofelements of titanium, zirconium, hafnium, and tantalum.
 10. Thesemiconductor device according to claim 9, wherein the second region andthe third region each comprise a region comprising an alloy.
 11. Anelectronic device comprising: the semiconductor device according toclaim 9; a housing; and a speaker. 12-15. (canceled)